Memory device and signal processing circuit

ABSTRACT

A memory device which can keep a stored logic state even when the power is off is provided. A signal processing circuit including the memory device, which achieves low power consumption by stopping supply of power, is provided. A memory device includes a logic circuit including a first node and a second node, a first memory circuit connected to the first node, a second memory circuit connected to the second node, and a precharge circuit connected to the first node, the second node, the first memory circuit, and the second memory circuit. When reading data is performed, the precharge circuit outputs a precharge potential to the first node and the second node. The first memory circuit and the second memory circuit each include a transistor in which a channel is formed in an oxide semiconductor film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/473,710, filed May 17, 2012, now allowed, which claims the benefit ofa foreign priority application filed in Japan as Serial No. 2011-113359on May 20, 2011, both of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a memory device utilizing a memoryelement, a manufacturing method thereof, and a driving method thereof.Further, the present invention relates to a signal processing circuitincluding the memory device.

2. Description of the Related Art

In recent years, with the widespread use of electronic devices such aspersonal computers and mobile phones, demand for higher performance ofelectronic devices has been increased. In order to achieve higherperformance of such electronic devices, higher performance of memorieshas been particularly required in addition to higher-speed operation ofinterfaces, improvement in processing performance of external devices,and the like.

The “memory” used here includes, in its category, not only a main memoryfor storing data and program but also a register, a cache memory, andthe like used in a signal processing circuit such as a centralprocessing unit (CPU). A register is provided to temporarily hold datafor carrying out arithmetic processing, holding a program executionstate, or the like. In addition, a cache memory is located between anarithmetic circuit and a main memory in order to reduce access to thelow-speed main memory and speed up the arithmetic processing. In amemory device such as a register or a cache memory, writing of dataneeds to be performed at higher speed than in a main memory. Thus, ingeneral, a flip-flop or the like is used as a register, and a volatilememory circuit such as a static random access memory (SRAM) is used as acache memory.

In order to reduce power consumption, a method for temporarily stoppingsupply of a power supply voltage to a signal processing circuit in aperiod during which data is not input and output has been suggested. Inthat method, a non-volatile memory circuit is located in the peripheryof a volatile memory circuit such as a register or a cache memory, andthe data is temporarily stored in the non-volatile memory circuit. Thus,in the signal processing circuit, the data stored in the register, thecache memory, or the like can be held even while a supply of powersupply voltage is stopped (for example, see Patent Document 1).

In addition, in the case where supply of the power supply voltage isstopped in a signal processing circuit for a long time, data in avolatile memory circuit may be transferred to an external memory devicesuch as a hard disk or a flash memory before supply of the power supplyvoltage is stopped, in which case the data can be prevented from beingerased.

REFERENCE

[Patent Document 1] Japanese Published Patent Application No. H10-078836

SUMMARY OF THE INVENTION

In such a signal processing circuit disclosed in Patent Document 1, inthe case of using a method for storing data of a volatile memory circuitin an external memory device while supply of power is stopped, it takestime to restore data from the external memory device to the volatilememory circuit after the supply of power is restarted. Therefore, such asignal processing circuit is not suitable in the case where supply ofpower is stopped for a short time for the purpose of a reduction inpower consumption.

In view of the above problem, an object of one embodiment of the presentinvention is to provide a memory device which can keep a stored logicstate even when the supply of power is stopped. Another object is toprovide a signal processing circuit including the memory device, whichachieves low power consumption by stopping supply of power.

A memory device according to one embodiment of the present inventionincludes a memory element including a logic circuit, a first memorycircuit, and a second memory circuit, and a precharge circuit. Aspecific structure thereof is described below.

The memory device according to one embodiment of the present inventionincludes a logic circuit including a first node and a second node, afirst memory circuit connected to the first node, a second memorycircuit connected to the second node, and a precharge circuit connectedto the first node, the second node, the first memory circuit, and thesecond memory circuit. The first memory circuit and the second memorycircuit each include a transistor in which a cannel is formed in anoxide semiconductor film and a capacitor. The precharge circuit outputsa precharge potential to the first node and the second node.

While power is supplied to the memory device, data is held in the firstnode and the second node of the logic circuit. Before the supply ofpower is stopped, the data which has been held in the first node and thesecond node of the logic circuit is held in the first memory circuit andthe second memory circuit connected to the first node and the secondnode, respectively.

The transistor included in each of the first memory circuit and thesecond memory circuit preferably has low off-state current.Specifically, the off-state current density is preferably less than orequal to 100 zA/μm, more preferably less than or equal to 10 zA/μm. Asthe transistor with low off-state current, it is preferable to use atransistor in which a channel is formed in a layer or a substrate formedusing a semiconductor with a larger bandgap than silicon. As an exampleof a semiconductor with a bandgap of greater than or equal to 2 eV,preferably greater than or equal to 2.5 eV, more preferably greater thanor equal to 3 eV, an oxide semiconductor can be given. A transistor inwhich a channel is formed in an oxide semiconductor has a characteristicof extremely low off-state current.

Therefore, such a transistor is used for each of the first memorycircuit and the second memory circuit, whereby in the case where thetransistor is off, a potential can be held by a capacitor connected tothe transistor for a long period. In addition, even in the case wheresupply of power is stopped, a logic state of the logic circuit can beheld in the first memory circuit and the second memory circuit. Withsuch a memory element, a memory device which can keep a stored logicstate even when the power is off can be provided.

Further, it is not necessary to transfer data held in the memory deviceto another memory device before supply of power is stopped; therefore,the supply of power can be stopped in a short time.

The oxide semiconductor film includes two or more elements selected fromindium, gallium, tin, and zinc.

The memory device according to one embodiment of the present inventionis provided with the precharge circuit, and the logic circuit, the firstmemory circuit, and the second memory circuit are connected to theprecharge circuit. When the supply of power to the memory device isrestarted and the data stored in the first memory circuit and the secondmemory circuit is restored to the logic circuit, the precharge potentialoutput from the precharge circuit is supplied to the first node wherethe logic circuit and the first memory circuit are connected to eachother and the second node where the logic circuit and the second memorycircuit are connected to each other. After that, the transistorsincluded in the first memory circuit and the second memory circuit areturned on. Thus, the potentials of the first node and the second node ofthe logic circuit vary depending on the potentials held in the firstmemory circuit and the second memory circuit, and can be set to thepotentials held before the supply of power is stopped. Therefore, thedata can be restored from the first memory circuit and the second memorycircuit to the first node and the second node of the logic circuit in ashort time.

With the use of the memory device according to one embodiment of thepresent invention for a signal processing circuit, power consumption canbe reduced in the case where supply of power is stopped for a shorttime.

According to one embodiment of the present invention, a memory devicewhich can keep a stored logic state even when the power is off can beprovided. With the memory device, a signal processing circuit can beprovided in which power consumption can be reduced by stopping thesupply of power.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a circuit diagram of a memory device;

FIG. 2 is a timing chart showing operation of a memory device;

FIG. 3 is a timing chart showing operation of a memory device;

FIG. 4 is a circuit diagram of a memory device;

FIG. 5 is a circuit diagram of a memory cell array;

FIG. 6 is a timing chart showing operation of a memory device;

FIGS. 7A to 7E illustrate a method for manufacturing a memory device;

FIGS. 8A to 8D illustrate a method for manufacturing a memory device;

FIGS. 9A to 9D illustrate a method for manufacturing a memory device;

FIGS. 10A and 10B illustrate a method for manufacturing a memory device;

FIGS. 11A to 11C are cross-sectional views of transistors;

FIGS. 12A to 12E each illustrate a crystal structure of an oxidematerial;

FIGS. 13A to 13C illustrate a crystal structure of an oxide material;

FIGS. 14A to 14C illustrate a crystal structure of an oxide material;

FIGS. 15A and 15B each illustrate a crystal structure of an oxidematerial;

FIG. 16 shows gate voltage dependence of mobility obtained bycalculation;

FIGS. 17A to 17C each show gate voltage dependence of drain current andmobility obtained by calculation;

FIGS. 18A to 18C each show gate voltage dependence of drain current andmobility obtained by calculation;

FIGS. 19A to 19C each show gate voltage dependence of drain current andmobility obtained by calculation;

FIGS. 20A and 20B each illustrate a cross-sectional structure of atransistor used in calculation;

FIGS. 21A and 21B are a top view and a cross-sectional view of atransistor;

FIGS. 22A and 22B are graphs each showing characteristics of atransistor;

FIG. 23 is a graph showing characteristics of a transistor;

FIG. 24 is a block diagram of a signal processing circuit; and

FIGS. 25A to 25F each illustrate an electronic device.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. Note that thepresent invention is not limited to the following description and itwill be readily appreciated by those skilled in the art that the modesand details of the present invention can be modified in various wayswithout departing from the spirit and scope thereof. Therefore, thepresent invention should not be interpreted as being limited to thedescription in the following embodiments.

Note that functions of a “source” and a “drain” may be switched in thecase where transistors of different polarities are employed or in thecase where the direction of a current flow changes in a circuitoperation, for example. Therefore, the terms “source” and “drain” can beused to denote the drain and the source, respectively, in thisspecification.

Voltage refers to a potential difference between a given potential and areference potential (e.g., a ground potential) in many cases.Accordingly, in this specification, voltage, a potential, and apotential difference can be referred to as a potential, voltage, and avoltage difference, respectively.

The term “over” or “below” does not necessarily mean that a component isplaced “directly on” or “directly under” another component. For example,the expression “a gate electrode over a gate insulating layer” can meanthe case where a component is placed between the gate insulating layerand the gate electrode.

The position, size, range, or the like of each component illustrated indrawings and the like is not accurately represented in some cases foreasy understanding. Therefore, the disclosed invention is notnecessarily limited to the position, size, range, or the like asdisclosed in the drawings and the like.

Ordinal numbers such as “first”, “second”, and “third” are used in orderto avoid confusion among components.

Embodiment 1

A memory element and a memory device according to one embodiment of thepresent invention will be described with reference to FIG. 1. FIG. 1illustrates a circuit configuration of a memory device 100.

<Structure of Memory Device>

The memory device 100 in FIG. 1 includes a memory element 110 and aprecharge circuit 108.

The memory element 110 includes a logic circuit 101, a memory circuit102, and a memory circuit 103. The memory element 110 may include aswitch 106 and a switch 107 in addition to the above circuits. A mainpower supply is set to a first power supply potential V1 (notillustrated). Note that in some circuit diagrams, “OS” (abbreviation ofan oxide semiconductor) is written besides a transistor in order toindicate that the transistor includes an oxide semiconductor.

The logic circuit 101 includes four transistors: two p-channeltransistors 111 and 112 and two n-channel transistors 113 and 114. Thetransistors 111 and 113 form an inverter and the transistors 112 and 114form an inverter. An input terminal of one of the inverters and anoutput terminal of the other of the inverters are cross-connected and anoutput terminal of one of the inverters and an input terminal of theother of the inverters are cross-connected, so that a flip flop havingtwo stable states is obtained.

In this specification and the like, the inverter including thetransistor 111 and the transistor 113 is referred to as a first invertercircuit, and the inverter including the transistor 112 and thetransistor 114 is referred to as a second inverter circuit. The inputterminal of the second inverter circuit, the output terminal of thefirst inverter circuit, and a first terminal of the switch 106 areelectrically connected to one another, and the connection point isreferred to as a node O. The input terminal of the first invertercircuit, the output terminal of the second inverter circuit, and a firstterminal of the switch 107 are electrically connected to one another,and the connection point is referred to as a node P. A node where one ofa source and a drain of the transistor 113 and one of a source and adrain of the transistor 114 are connected to each other is referred toas a node Q of the logic circuit 101, and a node where one of a sourceand a drain of the transistor 111 and one of a source and a drain of thetransistor 112 are connected to each other is referred to as a node R ofthe logic circuit 101. In addition, a second power supply potential V2(e.g., VSS) is input to the node Q, and a third power supply potentialV3 (e.g., VDD) is input to the node R.

The memory circuit 102 includes a transistor 115 and a capacitor 116.Here, one of a source and a drain of the transistor 115 is connected tothe node O of the logic circuit 101, and the other of the source and thedrain of the transistor 115 is connected to one of a pair of electrodesof the capacitor 116. A point where the transistor 115 and the capacitor116 are connected to each other is referred to as a node M. A controlsignal S2 is input to a gate of the transistor 115.

The memory circuit 103 includes a transistor 117 and a capacitor 118.Here, one of a source and a drain of the transistor 117 is connected tothe node P of the logic circuit 101, and the other of the source and thedrain of the transistor 117 is connected to one of a pair of electrodesof the capacitor 118. A point where the transistor 117 and the capacitor118 are connected to each other is referred to as a node N. The controlsignal S2 is input to a gate of the transistor 117.

Here, the transistor 115 and the transistor 117 each preferably have lowoff-state current. Specifically, the off-state current density ispreferably less than or equal to 100 zA/μm, more preferably less than orequal to 10 zA/μm. As the transistor with low off-state current, it ispreferable to use a transistor whose channel is formed in a layer or asubstrate formed using a semiconductor with a larger bandgap thansilicon. As an example of a semiconductor whose bandgap is greater thanor equal to 2 eV, preferably greater than or equal to 2.5 eV, morepreferably greater than or equal to 3 eV, an oxide semiconductor can begiven. A transistor whose channel is formed in an oxide semiconductorhas a characteristic of extremely low off-state current.

Therefore, when a transistor whose channel is formed in an oxidesemiconductor is used as the transistor 115, a potential of the node Mcan be kept for a long time with the transistor 115 being in an offstate. Similarly, when a transistor whose channel is formed in an oxidesemiconductor is used as the transistor 117, a potential of the node Ncan be kept for a long time with the transistor 117 being in an offstate.

In the case where an In—Sn—Zn—O-based material is used as an oxidesemiconductor material, the field-effect mobility of the transistor canbe 30 cm²/Vsec or higher, preferably 40 cm²/Vsec or higher, morepreferably 60 cm²/Vsec or higher, so that the memory circuit 102 and thememory circuit 103 can operate at high speed.

The switch 106 includes a transistor 123. A first terminal of the switch106 corresponds to one of a source and a drain of the transistor 123, asecond terminal thereof corresponds to the other of the source and thedrain of the transistor 123, and a third terminal thereof corresponds toa gate of the transistor 123. The first terminal of the switch 106 isconnected to the node O of the logic circuit 101. Data D is input to thesecond terminal of the switch 106. The case where an n-channeltransistor is used as the switch 106 is described; however, a p-channeltransistor may be used. Alternatively, the switch 106 may be acombination of an n-channel transistor and a p-channel transistor. Forexample, the switch 106 may be an analog switch.

The switch 107 includes a transistor 124. A first terminal of the switch107 corresponds to one of a source and a drain of the transistor 124, asecond terminal thereof corresponds to the other of the source and thedrain of the transistor 124, and a third terminal thereof corresponds toa gate of the transistor 124. The first terminal of the switch 107 isconnected to the node P of the logic circuit 101. Data DB is input tothe second terminal of the switch 107. The case where an n-channeltransistor is used as the switch 107 is described; however, a p-channeltransistor may be used. Alternatively, the switch 107 may be acombination of an n-channel transistor and a p-channel transistor. Forexample, the switch 107 may be an analog switch.

A control signal S1 is input to the third terminal of the switch 106 andthe third terminal of the switch 107. When the control signal S1 isinput to the third terminal of the switch 106, conduction ornon-conduction between the first terminal and the second terminal (on oroff state of the transistor 123) is selected. Similarly, when thecontrol signal S1 is input to the third terminal of the switch 107,conduction or non-conduction between the first terminal and the secondterminal (on or off state of the transistor 124) is selected.

The precharge circuit 108 includes a transistor 125, a transistor 126,and a transistor 127. One of a source and a drain of the transistor 125and one of a source and a drain of the transistor 126 are connected tothe node O of the logic circuit 101. The other of the source and thedrain of the transistor 125 and one of a source and a drain of thetransistor 127 are connected to the node P of the logic circuit 101. Aprecharge potential V_(PRE) (e.g., VDD/2) is supplied from the other ofthe source and the drain of the transistor 126 and the other of thesource and the drain of the transistor 127. A control signal S3 is inputto gates of the transistors 125, 126, and 127.

In this embodiment, the transistors 111 and 112 are p-channeltransistors, and the transistors 115, 117, and 123 to 127 are n-channeltransistors; however, one embodiment of the present invention is notlimited thereto, and the conductivity types of the transistors can beset as appropriate.

<Driving Method 1 of Memory Device>

Next, a driving method of the memory device 100 in FIG. 1 will bedescribed with reference to a timing chart in FIG. 2.

In the timing chart in FIG. 2, V1 represents a first power supplypotential (a main power supply); S1, a potential of the control signalS1; S2, a potential of the control signal S2; S3, a potential of thecontrol signal S3; O, a potential of the node O of the logic circuit101; P, a potential of the node P of the logic circuit 101; V2, a secondpower supply potential; V3, a third power supply potential; M, apotential of the node M; N, a potential of the node N; D, a potential ofthe data D; and DB, a potential of the data DB. The description is madeby referring to a low-level potential (also referred to as a firstpotential) as VSS, a precharge potential V_(PRE) as (VDD/2), and ahigh-level potential (also referred to as a second potential) as VDD.The case where the data D has the high-level potential and the data DBhas the low-level potential is described; however, the data D may havethe low-level potential and the data DB may have the high-levelpotential.

A period 1 is a period for writing data to the logic circuit 101. In theperiod 1, the high-level potential is supplied to the third terminals ofthe switches 106 and 107 as the control signal S1. Thus, electricalcontinuity between the first terminal and the second terminal isestablished in each of the switches 106 and 107. A potential of the dataD (high-level potential) is supplied to the input terminal of the secondinverter circuit through the switch 106, so that the transistor 114 isturned on. In addition, a potential of the data DB (low-level potential)is supplied to the input terminal of the first inverter circuit throughthe switch 107, so that the transistor 111 is turned on.

The low-level potential is supplied to the node Q as the second powersupply potential V2, and the high-level potential is supplied to thenode R as the third power supply potential V3.

Thus, the logic circuit 101 can be activated, and the data D and thedata DB can be held in the node O and the node P. After that, thelow-level potential is supplied to the third terminals of the switches106 and 107 as the control signal S1, whereby electrical discontinuitybetween the first terminal and the second terminal is established ineach of the switches 106 and 107.

A period 2 is a period for writing the data D and data DB written to thelogic circuit 101 to the memory circuit 102 and the memory circuit 103,respectively. In the period 2, the high-level potential is supplied tothe gates of the transistors 115 and 117 as the control signal S2, sothat the transistors 115 and 117 are turned on. Thus, the potentials ofthe data D and data DB held in the node O and the node P of the logiccircuit 101 are supplied to the node M and the node N, respectively.After that, the low-level potential is supplied to the gates of thetransistors 115 and 117 as the control signal S2, so that thetransistors 115 and 117 are turned off.

A period 3 is a period for stopping supply of power. In the period 3,the first power supply potential V1 is changed to the low-levelpotential, so that supply of power to the memory device 100 is stopped.At the same time, the third power supply potential V3 is changed to thelow-level potential.

The potentials of the nodes O and P of the logic circuit 101 cannot beheld because the supply of power is stopped.

In one embodiment of the present invention, a transistor with lowoff-state current is used as each of the transistors 115 and 117. Atransistor in which a channel is formed in an oxide semiconductor filmcan be used as the transistor with low off-state current. Such atransistor has a characteristic of extremely low off-state current.Therefore, even when the transistor 115 and the transistor 117 are off,a potential held by the capacitor 116 (potential of the node M) and apotential held by the capacitor 118 (potential of the node N) can beheld for a long time. In other words, after the supply of power isstopped, the potentials which have been held in the nodes O and P of thelogic circuit 101 can be held in the nodes M and N.

After that, the first power supply potential V1 is changed to thehigh-level potential, so that the supply of power to the memory device100 is restarted. At the same time, the precharge potential V_(PRE) (athird potential (e.g., VDD/2) between the first potential and the secondpotential) is supplied to the node Q as the second power supplypotential V2, and the third potential is supplied to the node R as thethird power supply potential V3.

A period 4 is a period for restoring the data D and data DB held in thememory circuit 102 and the memory circuit 103 to the node O and the nodeP of the logic circuit 101. First, the high-level potential is suppliedto the gates of the transistors 125, 126, and 127 as the control signalS3, so that the transistors 125, 126, and 127 are turned on. Thus, theprecharge potential V_(PRE) (the third potential (e.g., VDD/2) betweenthe first potential and the second potential) is supplied from the oneof the source and the drain of the transistor 126 and the one of thesource and the drain of the transistor 127 to the node O and the node Pof the logic circuit 101, so that the potentials of the nodes O and Pbecome the third potentials (e.g., VDD/2). After that, the low-levelpotential is supplied to the gates of the transistors 125, 126, and 127as the control signal S3, so that the transistors 125, 126, and 127 areturned off.

Next, the high-level potential is supplied to the gates of thetransistors 115 and 117 as the control signal S2, so that thetransistors 115 and 117 are turned on. Thus, the potentials of the nodesO and P of the logic circuit 101 vary. For example, in the case wherethe high-level potential is held in the memory circuit 102 and thelow-level potential is held in the memory circuit 103, the potential ofthe node O of the logic circuit 101 gradually increases and thepotential of the node P of the logic circuit 101 gradually decreases.After the difference between the potential of the node O and thepotential of the node P becomes ΔV, the low-level potential is suppliedto the node Q as the second power supply potential V2 and the high-levelpotential is supplied to the node R as the third power supply potentialV3. Specifically, ΔV is represented by the following formula (1).

$\begin{matrix}{{\Delta\; V} = {\frac{C_{s}}{C_{s} + C}V}} & (1)\end{matrix}$

V represents the high-level potential held in the memory circuit 102 orthe memory circuit 103; C_(s), capacitance of the capacitor 116 or thecapacitor 118; and C, parasitic capacitance of a wiring (also referredto as a bit line) connecting the transistor 126 and the transistor 115or parasitic capacitance of a wiring (also referred to as an invertedbit line) connecting the transistor 127 and the transistor 117. Notethat in this embodiment, the high-level potential is held in the node M;therefore, V represents a potential held in the node M of the memorycircuit 102, C_(s) represents capacitance of the capacitor 116, and Crepresents parasitic capacitance of the wiring connecting the transistor126 and the transistor 115.

After the difference between the potential of the node O and thepotential of the node P becomes ΔV, the low-level potential is suppliedto the node Q as the second power supply potential V2 and the high-levelpotential is supplied to the node R as the third power supply potentialV3, whereby the potentials of the nodes O and P can be effectivelyamplified. Thus, the potential of the node O of the logic circuit 101becomes the high-level potential and the potential of the node P of thelogic circuit 101 becomes the low-level potential.

Thus, the logic circuit 101 can be activated, and the data D and thedata DB can be held in the node O and the node P again. After that, thelow-level potential is supplied to the gates of the transistors 115 and117 as the control signal S2, whereby the transistors 115 and 117 areturned off.

A period 5 is a period for reading the data held in the nodes O and P ofthe logic circuit 101. In the period 5, the high-level potential issupplied to the third terminals of the switches 106 and 107 as thecontrol signal S1, so that electrical continuity between the firstterminal and the second terminal is established in each of the switches106 and 107. The data D held in the node O of the logic circuit 101 canbe read through the switch 106, and the data DB held in the node P ofthe logic circuit 101 can be read through the switch 107. After readingis completed, the low-level potential is supplied to the third terminalsof the switches 106 and 107 as the control signal S1, whereby electricaldiscontinuity between the first terminal and the second terminal isestablished in each of the switches 106 and 107.

The foregoing has described the driving method of the memory device 100.

In a memory device of one embodiment of the present invention, a memorycircuit including a transistor with low off-state current is provided ina memory element. A transistor in which a channel is formed in an oxidesemiconductor film can be used as the transistor with low off-statecurrent. Such a transistor has a characteristic of extremely lowoff-state current. Thus, even when the transistor is off, a potentialcan be held in a capacitor connected to the transistor for a long time.Therefore, even after supply of power is stopped, a logic state of thelogic circuit included in the memory element can be held. With the useof a plurality of such memory elements, a memory device which can keep astored logic state even when the power is off can be provided.

In the memory device according to one embodiment of the presentinvention, the data D and data DB held in the logic circuit 101 arerespectively held in the memory circuit 102 and the memory circuit 103which are connected to the logic circuit 101 before the supply of poweris stopped. Accordingly, since it is not necessary to transfer the dataheld in the memory device to another memory device before the supply ofpower is stopped, the supply of power can be stopped in a short time.

In the memory device according to one embodiment of the presentinvention, the precharge circuit connected to the logic circuit 101, thememory circuit 102, and the memory circuit 103 is provided. When thesupply of power is restarted and data held in the memory circuits 102and 103 is restored to the logic circuit 101, the precharge potentialV_(PRE) is supplied from the precharge circuit to the node O where thelogic circuit 101 and the memory circuit 102 are connected to each otherand the node P where the logic circuit 101 and the memory circuit 103are connected to each other. Thus, the potentials of the nodes O and Pof the logic circuit 101 vary depending on the potentials held in thememory circuits 102 and 103, and the potentials of the nodes O and P canbe set to the potentials held before the supply of power is stopped.Therefore, the data can be restored from the memory circuits 102 and 103to the nodes O and P of the logic circuit 101 in a short time.

With the use of the memory device according to one embodiment of thepresent invention for a signal processing circuit, power consumption canbe reduced in the case where supply of power is stopped for a shorttime.

<Driving Method 2 of Memory Device>

Next, another driving method of the memory device 100 in FIG. 1 will bedescribed with reference to a timing chart in FIG. 3.

A period 1 is a period for writing data to the logic circuit 101, thememory circuit 102, and the memory circuit 103. In the period 1, thehigh-level potential is supplied to the gates of the transistors 115 and117 as the control signal S2. Thus, the transistors 115 and 117 are on.After that, the high-level potential is supplied to the third terminalsof the switches 106 and 107 as the control signal S1. Thus, electricalcontinuity between the first terminal and the second terminal isestablished in each of the switches 106 and 107. A potential of the dataD (high-level potential) is supplied to the input terminal of the secondinverter circuit through the switch 106, so that the transistor 114 isturned on. In addition, a potential of the data DB (low-level potential)is supplied to the input terminal of the first inverter circuit throughthe switch 107, so that the transistor 111 is turned on.

The low-level potential is supplied to the node Q as the second powersupply potential V2, and the high-level potential is supplied to thenode R as the third power supply potential V3.

Thus, the logic circuit 101 can be activated, and the data D and thedata DB can be held in the node O and the node P. At this time, sincethe transistors 115 and 117 are on, the potentials of the data D anddata DB held in the node O and node P of the logic circuit 101 can besupplied to the node M and the node N through the transistor 115 and thetransistor 117, respectively.

After that, the low-level potential is supplied to the third terminalsof the switches 106 and 107 as the control signal S1, whereby electricaldiscontinuity between the first terminal and the second terminal isestablished in each of the switches 106 and 107. Further, when thecontrol signal S2 is changed to the low-level potential, the transistors115 and 117 are turned off.

By the driving method of a memory device in FIG. 3, the data D and thedata DB can be held in the memory circuit 102 and the memory circuit 103in a short time as compared to the case where the data D and the data DBare held in the logic circuit 101 and then held in the memory circuit102 and the memory circuit 103, respectively.

A period 2 is a period for stopping supply of power. In the period 2,the first power supply potential V1 is changed to the low-levelpotential, so that supply of power to the memory device 100 is stopped.At the same time, the third power supply potential V3 is changed to thelow-level potential.

The potentials of the nodes O and P of the logic circuit 101 cannot beheld because the supply of power is stopped.

In one embodiment of the present invention, a transistor with lowoff-state current is used as each of the transistors 115 and 117. Atransistor in which a channel is formed in an oxide semiconductor filmcan be used as the transistor with low off-state current. Such atransistor has a characteristic of extremely low off-state current.Therefore, even when the transistor 115 and the transistor 117 are off,a potential held by the capacitor 116 (potential of the node M) and apotential held by the capacitor 118 (potential of the node N) can beheld for a long time. In other words, after the supply of power isstopped, the potentials which have been held in the nodes O and P of thelogic circuit 101 can be held in the nodes M and N.

After that, the first power supply potential V1 is changed to thehigh-level potential, so that the supply of power to the memory device100 is restarted. At the same time, the precharge potential V_(PRE) (thethird potential (e.g., VDD/2) between the first potential and the secondpotential) is supplied to the node Q as the second power supplypotential V2, and the third potential is supplied to the node R as thethird power supply potential V3.

A period 3 is a period for restoring the data D and data DB held in thememory circuit 102 and the memory circuit 103 to the node O and the nodeP of the logic circuit 101. First, the high-level potential is suppliedto the gates of the transistors 125, 126, and 127 as the control signalS3, so that the transistors 125, 126, and 127 are turned on. Thus, theprecharge potential V_(PRE) (the third potential (e.g., VDD/2) betweenthe first potential and the second potential) is supplied from the oneof the source and the drain of the transistor 126 and the one of thesource and the drain of the transistor 127 to the node O and the node Pof the logic circuit 101, so that the potentials of the nodes O and Pbecome the third potentials (e.g., VDD/2). After that, the low-levelpotential is supplied to the gates of the transistors 125, 126, and 127as the control signal S3, so that the transistors 125, 126, and 127 areturned off.

Next, the high-level potential is supplied to the gates of thetransistors 115 and 117 as the control signal S2, so that thetransistors 115 and 117 are turned on. Thus, the potentials of the nodesO and P of the logic circuit 101 vary. For example, in the case wherethe high-level potential is held in the memory circuit 102 and thelow-level potential is held in the memory circuit 103, the potential ofthe node O of the logic circuit 101 gradually increases and thepotential of the node P of the logic circuit 101 gradually decreases.After the difference between the potential of the node O and thepotential of the node P becomes ΔV, the low-level potential is suppliedto the node Q as the second power supply potential V2 and the high-levelpotential is supplied to the node R as the third power supply potentialV3. Specifically, ΔV is represented by the following formula (1).

$\begin{matrix}{{\Delta\; V} = {\frac{C_{s}}{C_{s} + C}V}} & (1)\end{matrix}$

After the difference between the potential of the node O and thepotential of the node P becomes ΔV, the low-level potential is suppliedto the node Q as the second power supply potential V2 and the high-levelpotential is supplied to the node R as the third power supply potentialV3, whereby the potentials of the nodes O and P can be effectivelyamplified. Thus, the potential of the node O of the logic circuit 101becomes the high-level potential and the potential of the node P of thelogic circuit 101 becomes the low-level potential.

Thus, the logic circuit 101 can be activated, and the data D and thedata DB can be held in the node O and the node P again. After that, thelow-level potential is supplied to the gates of the transistors 115 and117 as the control signal S2, whereby the transistors 115 and 117 areturned off.

A period 4 is a period for reading the data held in the nodes O and P ofthe logic circuit 101. In the period 4, the high-level potential issupplied to the third terminals of the switches 106 and 107 as thecontrol signal S1, so that electrical continuity between the firstterminal and the second terminal is established in each of the switches106 and 107. The data D held in the node O of the logic circuit 101 canbe read through the switch 106, and the data DB held in the node P ofthe logic circuit 101 can be read through the switch 107. After readingis completed, the low-level potential is supplied to the third terminalsof the switches 106 and 107 as the control signal S1, whereby electricaldiscontinuity between the first terminal and the second terminal isestablished in each of the switches 106 and 107.

The foregoing has described the driving method of the memory device 100.

In a memory device of one embodiment of the present invention, a memorycircuit including a transistor with low off-state current is provided ina memory element. A transistor in which a channel is formed in an oxidesemiconductor film can be used as the transistor with low off-statecurrent. Such a transistor has a characteristic of extremely lowoff-state current. Thus, even when the transistor is off, a potentialcan be held in a capacitor connected to the transistor for a long time.Therefore, even after supply of power is stopped, a logic state of thelogic circuit included in the memory element can be held. With the useof a plurality of such memory elements, a memory device which can keep astored logic state even when the power is off can be provided.

In the memory device according to one embodiment of the presentinvention, the data D and data DB held in the logic circuit 101 arerespectively held in the memory circuit 102 and the memory circuit 103which are connected to the logic circuit 101 before the supply of poweris stopped. Accordingly, since it is not necessary to transfer the dataheld in the memory device to another memory device before the supply ofpower is stopped, the supply of power can be stopped in a short time.

In the memory device according to one embodiment of the presentinvention, the precharge circuit connected to the logic circuit 101, thememory circuit 102, and the memory circuit 103 is provided. When thesupply of power is restarted and data held in the memory circuits 102and 103 is restored to the logic circuit 101, the precharge potentialV_(PRE) is supplied from the precharge circuit to the node O where thelogic circuit 101 and the memory circuit 102 are connected to each otherand the node P where the logic circuit 101 and the memory circuit 103are connected to each other. Thus, the potentials of the nodes O and Pof the logic circuit 101 vary depending on the potentials held in thememory circuits 102 and 103, and the potentials of the nodes O and P canbe set to the potentials held before the supply of power is stopped.Therefore, the data can be restored from the memory circuits 102 and 103to the nodes O and P of the logic circuit 101 in a short time.

With the use of the memory device according to one embodiment of thepresent invention for a signal processing circuit, power consumption canbe reduced in the case where supply of power is stopped for a shorttime.

<Structure of Memory Device>

In FIG. 4, a memory device 150 partly different from the memory device100 in FIG. 1 is illustrated. The memory device 150 includes a memoryelement 160 and the precharge circuit 108. The memory element 160includes the logic circuit 101, the memory circuit 102, the memorycircuit 103, the switch 106, and the switch 107.

In the memory device 150 in FIG. 4, one of a source and a drain of thetransistor 125 and one of a source and a drain of the transistor 126 areconnected to a second terminal of the switch 106, and the other of thesource and the drain of the transistor 125 and one of a source and adrain of the transistor 127 which are included in the precharge circuit108 are connected to a second terminal of the switch 107. Otherstructures are the same as those of the memory device 100 in FIG. 1 andthus are not described in detail.

<Structure of Memory Cell Array>

Next, the case where a plurality of the memory elements 160 in FIG. 4form a memory cell array is illustrated in FIG. 5.

FIG. 5 is an example of a block diagram of a memory device including(m×n) memory elements 160. The case where the structure illustrated inFIG. 4 is used as the structure of the memory element 160 in FIG. 5 isdescribed.

A memory device 200 in FIG. 5 includes m (m is an integer of 2 or more)signal lines S1, m signal lines S2, n (n is an integer of 2 or more) bitlines BL, n inverted bit lines (/BL), a first power supply line V1, asecond power supply line V2, a third power supply line V3, a memory cellarray 210 having the memory elements 160 arranged in matrix of m rows(in the vertical direction)×n columns (in the horizontal direction), afirst driver circuit 211, and a second driver circuit 212. The firstdriver circuit 211 is connected to the n bit lines BL and the n invertedbit lines (/BL), and the second driver circuit 212 is connected to the msignal lines S1 and the m signal lines S2. The first power supply lineV1 (not illustrated) supplies power to the memory device 200, and thesecond power supply line V2 and the third power supply line V3 areconnected to the memory elements 160(1, 1) to 160(m, n). Note thatprecharge circuits 108_1 to 108 _(—) n are provided in the first drivercircuit 211.

Access to the memory elements 160(1, 1) to 160(m, n) is performedthrough the signal lines S1 and the signal lines S2. Data is written andread to/from the memory cells connected to the respective bit lines BLand inverted bit lines (/BL).

The first driver circuit 211 controls access through the bit lines BLand the inverted bit lines (/BL) to the memory cells in the horizontaldirection. On the other hand, the second driver circuit 212 controlsaccess through the signal lines S1 and the signal lines S2 to the memorycells in the vertical direction.

With the above operation, random access to the memory cell array 210 inFIG. 5 is possible.

Note that the case of using the memory element 160 in FIG. 4 isdescribed in FIG. 5; however, the memory element 110 in FIG. 1 also canbe used. In the case where the memory element 110 in FIG. 1 is used asthe memory element in the memory device, it is preferable that aprecharge circuit be not provided in the first driver circuit 211 butprovided in each of the memory elements 110 to form a memory cell array.

<Driving Method of Memory Device>

Next, a driving method of the memory device 200 in FIG. 5 will bedescribed with reference to a timing chart in FIG. 6.

In this embodiment, the case where data is written to an i-th row (i isa natural number of greater than or equal to 1 and less than or equal tom) of the memory cell array 210 in FIG. 5, supply of power is stopped,the supply of power is restarted, and the data is read will bedescribed. The timing chart in FIG. 6 illustrates operation of thememory elements 160(i, 1) to 160(i, n) in the i-th row.

A period 1 is a period for writing data to the logic circuits 101included in the memory elements 160(i, 1) to 160(i, n) in the i-th row.In the period 1, the high-level potential is supplied to the thirdterminals of the switch 106 and the switch 107 included in each of thememory elements 160(i, 1) to 160(i, n), as a control signal S1 _(—) i inthe i-th row. Thus, electrical continuity between the first terminal andthe second terminal is established in each of the switches 106 and 107.In each of the memory elements 160(i, 1) to 160(i, n), a potential ofthe data D (high-level potential) is supplied to the input terminal ofthe second inverter circuit through the switch 106, so that thetransistor 114 is turned on. In addition, a potential of the data DB(low-level potential) is supplied to the input terminal of the firstinverter circuit through the switch 107, so that the transistor 111 isturned on. Note that control signals S1 in rows except the i-th row, inwhich data is not written to the logic circuit 101, are set to thelow-level potential.

In each of the memory elements 160(i, 1) to 160(i, n), the low-levelpotential is supplied to the node Q as the second power supply potentialV2, and the high-level potential is supplied to the node R as the thirdpower supply potential V3.

Thus, the logic circuit 101 in each of the memory elements 160(i, 1) to160(i, n) can be activated, and the data D and the data DB can be heldin the node O and the node P. After that, the low-level potential issupplied to the third terminals of the switches 106 and 107 as thecontrol signal S1 _(—) i in the i-th row, whereby electricaldiscontinuity between the first terminal and the second terminal isestablished in each of the switches 106 and 107.

A period 2 is a period for writing the data D and data DB written to thelogic circuit 101 to the memory circuit 102 and the memory circuit 103,respectively, in each of the memory elements 160(i, 1) to 160(i, n) inthe i-th row. In the period 2, the high-level potential is supplied tothe gates of the transistors 115 and 117 as a control signal S2 _(—) iin the i-th row, so that the transistors 115 and 117 are turned on.Thus, the potentials of the data D and data DB held in the node O andthe node P of the logic circuit 101 are supplied to the node M and thenode N, respectively. After that, the low-level potential is supplied tothe gates of the transistors 115 and 117 as the control signal S2 in thei-th row, so that the transistors 115 and 117 are turned off. Note thatthe control signals S2 in the rows except the i-th row, in which thedata is not written to the logic circuit 101, are set to the low-levelpotentials.

A period 3 is a period for stopping supply of power. In the period 3,the first power supply potential V1 is changed to the low-levelpotential, so that the supply of power to the memory device 200 isstopped. At the same time, the third power supply potential V3 ischanged to the low-level potential.

The potentials of the nodes O and P of the logic circuit 101 in each ofthe memory elements 160(i, 1) to 160(i, n) in the i-th row cannot beheld because the supply of power is stopped.

In one embodiment of the present invention, a transistor with lowoff-state current is used as each of the transistors 115 and 117. Atransistor in which a channel is formed in an oxide semiconductor filmcan be used as the transistor with low off-state current. Such atransistor has a characteristic of extremely low off-state current.Therefore, even when the transistor 115 and the transistor 117 are off,a potential held by the capacitor 116 (potential of the node M) and apotential held by the capacitor 118 (potential of the node N) can beheld for a long time. In other words, after the supply of power isstopped, the potentials which have been held in the nodes O and P of thelogic circuit 101 can be held in the nodes M and N.

After that, the first power supply potential V1 is changed to thehigh-level potential, so that the supply of power to the memory device200 is restarted. At the same time, the precharge potential V_(PRE) (thethird potential (e.g., VDD/2) between the first potential and the secondpotential) is supplied to the node Q of the logic circuit 101 in each ofthe memory elements 160(i, 1) to 160(i, n) in the i-th row as the secondpower supply potential V2, and the third potential is supplied to thenode R as the third power supply potential V3.

A period 4 is a period for restoring the data D and data DB held in thememory circuit 102 and the memory circuit 103 in each of the memoryelements 160(i, 1) to 160(i, n) in the i-th row to the node O and thenode P of the logic circuit 101. First, the high-level potential issupplied to the gates of the transistors 125, 126, and 127 in the firstcolumn to the n-th column as the control signal S3 in the first columnto the n-th column, so that the transistors 125, 126, and 127 are turnedon. Thus, the precharge potential V_(PRE) (the third potential (e.g.,VDD/2) between the first potential and the second potential) is suppliedfrom the one of the source and the drain of the transistor 126 and theone of the source and the drain of the transistor 127 to the node O andthe node P of the logic circuit 101, so that the potentials of the nodesO and P become the third potentials (e.g., VDD/2). After that, thelow-level potential is supplied to the gates of the transistors 125,126, and 127 as the control signal S3 in the first column to the n-thcolumn, so that the transistors 125, 126, and 127 are turned off.

Next, the high-level potential is supplied to the gates of thetransistors 115 and 117 as the control signal S2 _(—) i in the i-th row,so that the transistors 115 and 117 are turned on. Thus, the potentialsof the nodes O and P of the logic circuit 101 vary. For example, in thecase where the high-level potential is held in the memory circuit 102and the low-level potential is held in the memory circuit 103, thepotential of the node O of the logic circuit 101 gradually increases andthe potential of the node P of the logic circuit 101 graduallydecreases. After the difference between the potential of the node O andthe potential of the node P becomes ΔV, the low-level potential issupplied to the node Q as the second power supply potential V2 and thehigh-level potential is supplied to the node R as the third power supplypotential V3. Specifically, ΔV is represented by the following formula(1).

$\begin{matrix}{{\Delta\; V} = {\frac{C_{s}}{C_{s} + C}V}} & (1)\end{matrix}$

After the difference between the potential of the node O and thepotential of the node P becomes ΔV, the low-level potential is suppliedto the node Q as the second power supply potential V2 and the high-levelpotential is supplied to the node R as the third power supply potentialV3, whereby the potentials of the nodes O and P can be effectivelyamplified. Thus, the potential of the node O of the logic circuit 101becomes the high-level potential and the potential of the node P of thelogic circuit 101 becomes the low-level potential.

Thus, the logic circuit 101 in each of the memory elements 160(i, 1) to160(i, n) in the i-th row can be activated, and the data D and the dataDB can be held in the node O and the node P again. After that, thelow-level potential is supplied to the gates of the transistors 115 and117 as the control signal S2 _(—) i in the i-th row, whereby thetransistors 115 and 117 are turned off.

A period 5 is a period for reading the data held in the node O and thenode P of the logic circuit 101 in each of the memory elements 160(i, 1)to 160(i, n) in the i-th row. In the period 5, the high-level potentialis supplied to the third terminals of the switches 106 and 107 as thecontrol signal in the i-th row, so that electrical continuity betweenthe first terminal and the second terminal is established in each of theswitches 106 and 107. The data D held in the node O of the logic circuit101 can be read through the switch 106, and the data DB held in the nodeP of the logic circuit 101 can be read through the switch 107.

In a memory device of one embodiment of the present invention, a memorycircuit including a transistor with low off-state current is provided ina memory element. A transistor in which a channel is formed in an oxidesemiconductor film can be used as the transistor with low off-statecurrent. Such a transistor has a characteristic of extremely lowoff-state current. Even when the transistor is off, a potential can beheld in a capacitor connected to the transistor for a long time.Therefore, even after supply of power is stopped, a logic state of thelogic circuit included in the memory element can be held. With the useof a plurality of such memory elements, a memory device which can keep astored logic state even when the power is off can be provided.

In the memory device according to one embodiment of the presentinvention, the data D and data DB held in the logic circuit 101 arerespectively held in the memory circuit 102 and the memory circuit 103which are connected to the logic circuit 101 before the supply of poweris stopped. Accordingly, since it is not necessary to transfer the dataheld in the memory device to another memory device before the supply ofpower is stopped, the supply of power can be stopped in a short time.

In the memory device according to one embodiment of the presentinvention, the precharge circuit connected to the logic circuit 101, thememory circuit 102, and the memory circuit 103 is provided. When thesupply of power is restarted and data held in the memory circuits 102and 103 is restored to the logic circuit 101, the precharge potentialV_(PRE) is supplied from the precharge circuit to the node O where thelogic circuit 101 and the memory circuit 102 are connected to each otherand the node P where the logic circuit 101 and the memory circuit 103are connected to each other. Thus, the potentials of the nodes O and Pof the logic circuit 101 vary depending on the potentials held in thememory circuits 102 and 103, and the potentials of the nodes O and P canbe set to the potentials held before the supply of power is stopped.Therefore, the data can be restored from the memory circuits 102 and 103to the nodes O and P of the logic circuit 101 in a short time.

With the use of the memory device according to one embodiment of thepresent invention for a signal processing circuit, power consumption canbe reduced in the case where supply of power is stopped for a shorttime.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 2

In this embodiment, an example of a manufacturing method of the memoryelement described in Embodiment 1 will be described with reference toFIGS. 7A to 7E, FIGS. 8A to 8D, FIGS. 9A to 9D, and FIGS. 10A and 10B.First, a manufacturing method of a transistor in a lower portion of thememory device will be described, and then, a manufacturing method of atransistor and a capacitor in an upper portion of the memory device willbe described. Note that in cross-sectional views illustrating amanufacturing process, A1-A2 is a cross section illustrating amanufacturing step of an n-channel transistor and B1-B2 is a crosssection illustrating a manufacturing step of a p-channel transistor.

<Manufacturing Method of Transistor in Lower Portion>

First, a substrate 300 over which a semiconductor film 304 is providedwith an insulating film 302 interposed therebetween is prepared (seeFIG. 7A).

As the substrate 300, for example, a single crystal semiconductorsubstrate or a polycrystalline semiconductor substrate containingsilicon, silicon carbide, or the like, or a compound semiconductorsubstrate containing silicon germanium, gallium arsenide, indiumphosphide, or the like can be used. Specific examples thereof are avariety of glass substrates that are used in the electronics industry,such as substrates of aluminosilicate glass, aluminoborosilicate glass,and barium borosilicate glass, a quartz substrate, a ceramic substrate,and a sapphire substrate.

The insulating film 302 is formed to have a single-layer structure or astacked-layer structure using silicon oxide, silicon oxynitride, siliconnitride, or the like. As a formation method of the insulating film 302,a thermal oxidation method, a CVD method, a sputtering method, or thelike can be used. The thickness of the insulating film 302 is greaterthan or equal to 1 nm and less than or equal to 100 nm, preferablygreater than or equal to 10 nm and less than or equal to 50 nm.

As the semiconductor film 304, a single crystal semiconductor materialor a polycrystalline semiconductor material of silicon, silicon carbide,or the like, or a compound semiconductor material of silicon germanium,gallium arsenide, indium phosphide, or the like can be used. Since thesemiconductor film 304 does not include an oxide semiconductor material,the semiconductor film 304 is also referred to as a semiconductormaterial other than an oxide semiconductor.

As the semiconductor film 304, a single crystal semiconductor materialof silicon or the like is preferably used because in that case, thelogic circuit 101, the switch 106, the switch 107, and the like whichare described in Embodiment 1 can operate at higher speed.

Alternatively, an SOI substrate can be used as the substrate 300 overwhich the semiconductor film 304 is provided with the insulating film302 interposed therebetween. Note that although the term “SOI substrate”generally means a substrate in which a silicon layer is provided on aninsulating surface, the term “SOI substrate” in this specification andthe like also includes a substrate in which a semiconductor filmincluding a material other than silicon is provided on an insulatingsurface. That is, the semiconductor film included in the “SOI substrate”is not limited to a silicon layer. Moreover, the SOI substrate alsoincludes a substrate having a structure in which a semiconductor film isprovided over an insulating substrate such as a glass substrate with aninsulating film interposed therebetween. In this embodiment, the case isdescribed in which an SOI substrate in which a silicon film is providedover a single crystal silicon substrate with a silicon oxide filminterposed therebetween is used as the substrate 300 over which thesemiconductor film 304 is provided with the insulating film 302interposed therebetween.

Next, the semiconductor film 304 is processed into an island shape, sothat semiconductor films 304 a and 304 b are formed (see FIG. 7B). Forthe processing, dry etching is preferably performed, but wet etching maybe performed. An etching gas and an etchant can be selected asappropriate depending on a material to be etched.

Next, gate insulating films 306 a and 306 b are formed so as to coverthe semiconductor films 304 a and 304 b (see FIG. 7B). The gateinsulating films 306 a and 306 b can be formed, for example, byperforming heat treatment (e.g., thermal oxidation treatment, thermalnitridation treatment, or the like) on surfaces of the semiconductorfilms 304 a and 304 b. High-density plasma treatment may be employedinstead of heat treatment. The high-density plasma treatment can beperformed using, for example, a mixed gas of a rare gas such as He, Ar,Kr, or Xe and any of oxygen, nitrogen oxide, ammonia, nitrogen, andhydrogen. Needless to say, the gate insulating films may be formed by aCVD method, a sputtering method, or the like.

The gate insulating films 306 a and 306 b can be formed using siliconoxide, silicon oxynitride, silicon nitride, hafnium oxide, aluminumoxide, tantalum oxide, or the like. Alternatively, the gate insulatingfilms may be formed using a material with a high dielectric constant (ahigh-k material) such as hafnium oxide, yttrium oxide, hafnium silicate(HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate to which nitrogen is added(HfSi_(x)O_(y)N_(z) (x>0, y>0, z>0)), or hafnium aluminate to whichnitrogen is added (HfAl_(x)O_(y)N_(z) (x>0, y>0, z>0)). The gateinsulating films are formed to have a single-layer structure or astacked-layer structure using any of the above materials. The thicknessof each of the gate insulating films 306 a and 306 b can be, forexample, greater than or equal to 1 nm and less than or equal to 100 nm,preferably greater than or equal to 10 nm and less than or equal to 50nm.

When the gate insulating films are thin as in the above description, aproblem of gate leakage due to a tunneling effect or the like is caused.In order to solve the problem of gate leakage, the above high-k materialis preferably used for the gate insulating films. With the use of ahigh-k material for the gate insulating films, the thickness of each ofthe gate insulating films can be increased to prevent gate leakage andelectric characteristics can be maintained. Note that a stackedstructure of a film including a high-k material and a film containingany of silicon oxide, silicon nitride, silicon oxynitride, siliconnitride oxide, aluminum oxide, and the like may be employed.

In this embodiment, a silicon oxide film is formed by oxidationtreatment, whereby the gate insulating films 306 a and 306 b are formed.

Next, an impurity element imparting n-type conductivity and an impurityelement imparting p-type conductivity are added to the semiconductorfilms 304 a and 304 b through the gate insulating films 306 a and 306 bin order to control the threshold voltages of the transistors (see FIG.7C). In the case where silicon is used for the semiconductor films 304 aand 304 b, for example, phosphorus, arsenic, or the like can be used asan impurity element imparting n-type conductivity. On the other hand,boron, aluminum, gallium, or the like can be used as an impurity elementimparting p-type conductivity. In this embodiment, boron is added to thesemiconductor film 304 a through the gate insulating film 306 a, so thatan impurity region 308 is formed, and phosphorus is added to thesemiconductor film 304 b through the gate insulating film 306 b, so thatan impurity region 310 is formed.

Next, a conductive film used for forming a gate electrode (including awiring formed using the same layer as the gate electrode) is formed overthe gate insulating films 306 a and 306 b and is processed, so that gateelectrodes 312 a and 312 b are formed (see FIG. 7D).

The conductive film used for the gate electrodes 312 a and 312 b can beformed using a metal material such as aluminum, copper, titanium,tantalum, or tungsten. The conductive film may be formed using asemiconductor material such as polycrystalline silicon. There is noparticular limitation on the method for forming the conductive film, anda variety of film formation methods such as an evaporation method, a CVDmethod, a sputtering method, and a spin coating method can be employed.The conductive film can be processed by etching with the use of a resistmask. In this embodiment, a tantalum nitride film and a tungsten filmare stacked by a sputtering method and processed, so that the gateelectrodes 312 a and 312 b are formed.

Next, an impurity element imparting n-type conductivity and an impurityelement imparting p-type conductivity are added to the semiconductorfilms 304 a and 304 b using the gate electrodes 312 a and 312 b as masksthrough the gate insulating films 306 a and 306 b (see FIG. 7E). In thisembodiment, phosphorus is added to the semiconductor film 304 a throughthe gate insulating film 306 a, so that impurity regions 314 a and 314 bare formed, and boron is added to the semiconductor film 304 b throughthe gate insulating film 306 b, so that impurity regions 316 a and 316 bare formed.

Next, sidewall insulating films 318 a to 318 d having a sidewallstructure are formed on side surfaces of the gate electrodes 312 a and312 b (see FIG. 8A). The sidewall insulating films 318 a to 318 d may beformed on the side surfaces of the gate electrodes 312 a and 312 b in aself-aligning manner, by forming an insulating film that covers the gateelectrodes 312 a and 312 b, and then processing the insulating film byanisotropic etching by a reactive ion etching (RIE) method. There is noparticular limitation on the insulating film; for example, theinsulating film can be formed using silicon oxide with favorable stepcoverage, which is formed by reaction of tetraethyl ortho-silicate(TEOS), silane, or the like with oxygen, nitrous oxide, or the like. Theinsulating film may be formed using silicon oxide formed by a lowtemperature oxidation (LTO) method. The insulating film can be formed bya thermal CVD method, a plasma enhanced CVD method, an atmosphericpressure CVD method, a bias ECRCVD method, a sputtering method, or thelike.

Next, an impurity element imparting n-type conductivity and an impurityelement imparting p-type conductivity are added to the semiconductorfilms 304 a and 304 b using the gate electrodes 312 a and 312 b and thesidewall insulating films 318 a to 318 d as masks through the gateinsulating films 306 a and 306 b (see FIG. 8B). In this embodiment,phosphorus is added to the semiconductor film 304 a through the gateinsulating film 306 a, so that impurity regions 320 a and 320 b areformed, and boron is added to the semiconductor film 304 b through thegate insulating film 306 b, so that impurity regions 322 a and 322 b areformed. The impurity element is preferably added so that the impurityregions 320 a and 320 b have higher concentration than the impurityregions 314 a and 314 b, and the impurity element is preferably added sothat the impurity regions 322 a and 322 b have higher concentration thanthe impurity regions 316 a and 316 b.

Through the above steps, an n-channel transistor and a p-channeltransistor can be manufactured using the substrate 300 including asemiconductor material other than an oxide semiconductor (see FIG. 8B).Such transistors are capable of high-speed operation. Thus, thetransistors are preferably applied to the logic circuit 101, the switch106, the switch 107, the precharge circuit 108, and the like because inthat case, the speed of operation thereof can be increased.

Next, an insulating film 324 is formed so as to cover the transistor 113and the transistor 111 (see FIG. 8C). The insulating film 324 can beformed using a material containing an inorganic insulating material suchas silicon oxide, silicon oxynitride, silicon nitride, or aluminumoxide. A material with a low dielectric constant (a low-k material) ispreferably used for the insulating film 324 because capacitance due tooverlap of electrodes or wirings can be sufficiently reduced. Note thata porous insulating film formed using such a material may be used as theinsulating film 324. The porous insulating film has a lower dielectricconstant than an insulating film with high density and thus makes itpossible to further reduce capacitance due to electrodes or wirings.Alternatively, the insulating film 324 can be formed using an organicinsulating material such as polyimide or acrylic. In this embodiment,the case where the insulating film 324 is formed using siliconoxynitride is described.

Next, after the insulating film 324 is formed, heat treatment isperformed to activate the impurity elements added to the semiconductorfilms 304 a and 304 b. The heat treatment is performed using anannealing furnace. Alternatively, a laser annealing method or a rapidthermal annealing (RTA) method can be used. The heat treatment isperformed at 400° C. to 600° C., typically 450° C. to 500° C. in anitrogen atmosphere for 1 to 4 hours. By this heat treatment, activationof the impurity elements is performed and hydrogen in the siliconoxynitride film of the insulating film 324 is released, so thathydrogenation of the semiconductor films 304 a and 304 b can beperformed.

Note that before or after each of the above steps, a step of forming anelectrode, a wiring, a semiconductor film, an insulating film, or thelike may be further performed. For example, an electrode, a wiring, orthe like for connecting the transistor in the lower portion and thetransistor in the upper portion is preferably formed. In addition, amultilayer wiring structure in which an insulating film and a conductivelayer are stacked may be employed as a wiring structure, so that ahighly-integrated memory device can be achieved.

<Manufacturing Method of Transistor in Upper Portion>

First, as treatment before formation of the transistor 115 and thecapacitor 116, a surface of the insulating film 324 is planarized (seeFIG. 8D). As the planarization treatment for the insulating film 324,etching treatment or the like can be employed instead of polishingtreatment such as chemical mechanical polishing (hereinafter, alsoreferred to as CMP treatment). CMP treatment and etching treatment maybe performed in combination. The surface of the insulating film 324 ispreferably planarized as much as possible in order to improvecharacteristics of the transistor 115.

Here, CMP treatment is a method of planarizing a surface of an object tobe processed with a combination of chemical and mechanical actions,using the surface as a reference. Specifically, CMP treatment is amethod in which a polishing cloth is attached to a polishing stage, thepolishing stage and an object to be processed are rotated or swung whilea slurry (an abrasive) is supplied between the object and the polishingcloth, and the surface of the object is polished by a chemical reactionbetween the slurry and the object and by action of mechanical polishingof the object with the polishing cloth.

The oxide semiconductor film is preferably formed over the surface ofthe insulating film 324 with an average surface roughness (R_(a)) ofless than or equal to 1 nm, preferably less than or equal to 0.3 nm,more preferably less than or equal to 0.1 nm. Note that R_(a) isobtained by three-dimension expansion of center line average surfaceroughness which is defined by JIS B 0601 so as to be applied to a plane.The R_(a) can be expressed as an “average value of the absolute valuesof deviations from a reference surface to a specific surface” and isdefined by the following formula (2).

$\begin{matrix}{{Ra} = {\frac{1}{S_{0}}{\int_{y_{1}}^{y_{2}}{\int_{x_{1}}^{x_{2}}{{{{f( {x,y} )} - Z_{0}}}\ {\mathbb{d}x}\ {\mathbb{d}y}}}}}} & (2)\end{matrix}$

In the above formula, S₀ represents the area of a plane to be measured(a quadrangular region which is defined by four points represented bycoordinates (x₁, y₁), (x₁, y₂), (x₂, y₁), and (x₂, y₂)), and Z₀represents the average height of the plane to be measured. Further,R_(a) can be measured using an atomic force microscope (AFM).

Next, an oxide semiconductor film 342 is formed over the planarizedsurface of the insulating film 324.

An oxide semiconductor to be used preferably contains at least indium(In) or zinc (Zn). In particular, In and Zn are preferably contained. Asa stabilizer for reducing variation in electric characteristics of atransistor including the oxide semiconductor, it is preferable that oneor more elements selected from gallium (Ga), tin (Sn), hafnium (Hf), andaluminum (Al) be contained.

As another stabilizer, one or plural kinds of lanthanoid such aslanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium(Lu) may be contained.

As the oxide semiconductor, for example, indium oxide, tin oxide, zincoxide, a two-component metal oxide such as an In—Zn-based oxide, aSn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, aSn—Mg-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide, athree-component metal oxide such as an In—Ga—Zn-based oxide (alsoreferred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide,a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide,an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-basedoxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, anIn—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide,an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-basedoxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, anIn—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide, a four-component metaloxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, anIn—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide can be used.

Note that here, for example, an In—Ga—Zn-based oxide means an oxidecontaining In, Ga, and Zn as its main component, and there is nolimitation on the ratio of In:Ga:Zn. Further, the In—Ga—Zn-based oxidemay contain a metal element other than In, Ga, and Zn.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=⅓:⅓:⅓) or In:Ga:Zn=2:2:1 (=⅖:⅖:⅕), or any of oxideswhose composition is in the neighborhood of the above compositions canbe used. Alternatively, an In—Sn—Zn-based oxide with an atomic ratio ofIn:Sn:Zn=1:1:1 (=⅓:⅓:⅓), In:Sn:Zn=2:1:3 (=⅓:⅙:½), or In:Sn:Zn=2:1:5(=¼:⅛:⅝), or any of oxides whose composition is in the neighborhood ofthe above compositions may be used.

However, the composition is not limited to those described above, and amaterial having an appropriate composition may be used depending onneeded semiconductor characteristics (such as mobility, thresholdvoltage, and variation). In order to obtain needed semiconductorcharacteristics, it is preferable that the carrier concentration, theimpurity concentration, the defect density, the atomic ratio of a metalelement to oxygen, the interatomic distance, the density, and the likebe set to appropriate values.

For example, it is relatively easy to obtain high mobility with anIn—Sn—Zn-based oxide. However, it is possible to obtain high mobilityeven with an In—Ga—Zn-based oxide by reducing the defect density in abulk.

Note that for example, the expression “the composition of an oxidecontaining In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1),is in the neighborhood of the composition of an oxide containing In, Ga,and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1)” means that a, b,and c satisfy the following relation: (a−A)²+(b−B)²+(c−C)²≦r², and r maybe 0.05, for example. The same applies to other oxides.

The oxide semiconductor may be either single crystal ornon-single-crystal. In the latter case, the oxide semiconductor may beeither amorphous or polycrystalline. Further, the oxide semiconductormay have either an amorphous structure including a portion havingcrystallinity or a non-amorphous structure.

In an oxide semiconductor in an amorphous state, a flat surface can beobtained with relative ease, so that when a transistor is manufacturedwith the use of the oxide semiconductor, interface scattering can bereduced, and relatively high mobility can be obtained with relativeease.

In an oxide semiconductor having crystallinity, defects in a bulk can befurther reduced and when a surface flatness is improved, and mobilityhigher than that of an oxide semiconductor in an amorphous state can beobtained. In order to improve the surface flatness, the oxidesemiconductor is preferably formed over a flat surface. Specifically,the oxide semiconductor may be formed over a surface with the averagesurface roughness (R_(a)) of less than or equal to 1 nm, preferably lessthan or equal to 0.3 nm, more preferably less than or equal to 0.1 nm.

In the case where an In—Zn-based oxide semiconductor material is used asthe oxide semiconductor film 342, a ratio of atoms of metal elements ofa target is In:Zn=50:1 to 1:2 in atomic ratio (In₂O₃:ZnO=25:1 to 1:4 inmolar ratio), preferably, In:Zn=20:1 to 1:1 in atomic ratio(In₂O₃:ZnO=10:1 to 1:2 in molar ratio), further preferably, In:Zn=15:1to 1.5:1 in atomic ratio (In₂O₃:ZnO=15:2 to 3:4 in molar ratio). Forexample, in a target used for formation of an In—Zn—O-based oxidesemiconductor which has an atomic ratio of In:Zn:O=X:Y:Z, the relationof Z>1.5X+Y is satisfied.

In the case of forming the oxide semiconductor film 342 using anIn—Ga—Zn-based oxide semiconductor material by a sputtering method, itis preferable to use an In—Ga—Zn-based oxide target having an atomicratio of In:Ga:Zn=1:1:1, 4:2:3, 3:1:2, 1:1:2, 2:1:3, or 3:1:4.

In the case of forming the oxide semiconductor film 342 using anIn—Sn—Zn-based oxide semiconductor material by a sputtering method, itis preferable to use an In—Sn—Zn-based oxide target having an atomicratio of In:Sn:Zn=1:1:1, 2:1:3, 1:2:2, or 20:45:35.

The relative density of the target is higher than or equal to 90% andlower than or equal to 100%, preferably higher than or equal to 95% andlower than or equal to 99.9%. With the use of the target with highrelative density, the oxide semiconductor film 342 can have highdensity.

The oxide semiconductor film 342 can be formed by a sputtering method, amolecular beam epitaxy method, an atomic layer deposition method, or apulsed laser deposition method. The thickness of the oxide semiconductorfilm 342 is greater than or equal to 5 nm and less than or equal to 100nm, preferably greater than or equal to 10 nm and less than or equal to30 nm.

The oxide semiconductor film 342 may be amorphous or may havecrystallinity. For example, the oxide semiconductor film is anon-single-crystal oxide including a phase which has a triangular,hexagonal, regular triangular, or regular hexagonal atomic arrangementwhen seen from the direction perpendicular to the a-b plane and in whichmetal atoms are arranged in a layered manner or metal atoms and oxygenatoms are arranged in a layered manner when seen from the directionperpendicular to the c-axis. Note that in this specification and thelike, an oxide semiconductor film including a c-axis aligned crystal iscalled a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film.

The CAAC-OS film is not a single crystal, but this does not mean thatthe CAAC-OS film is composed of only an amorphous component. Althoughthe CAAC-OS film includes a crystallized portion (crystalline portion),a boundary between one crystalline portion and another crystallineportion is not clear in some cases.

In the case where oxygen is included in the CAAC-OS film, nitrogen maybe substituted for part of oxygen included in the CAAC-OS film. Thec-axes of individual crystalline portions included in the CAAC-OS filmmay be aligned in one direction (e.g., a direction perpendicular to asurface of a substrate over which the CAAC-OS film is formed or asurface of the CAAC-OS film). Alternatively, the normals of the a-bplanes of the individual crystalline portions included in the CAAC-OSfilm may be aligned in one direction (e.g., a direction perpendicular toa surface of a substrate over which the CAAC-OS film is formed or asurface of the CAAC-OS film).

The CAAC-OS film becomes a conductor, a semiconductor, or an insulatordepending on its composition or the like. The CAAC-OS film transmits ordoes not transmit visible light depending on its composition or thelike.

As an example of a crystalline portion included in such a CAAC-OS film,there is a crystalline portion which is formed into a film shape and hasa triangular or hexagonal atomic arrangement when observed from thedirection perpendicular to a surface of the film or a surface of asubstrate over which the CAAC-OS film is formed, and in which metalatoms are arranged in a layered manner or metal atoms and oxygen atoms(or nitrogen atoms) are arranged in a layered manner when a crosssection of the film is observed.

Next, a formation method of the oxide semiconductor film 342 as aCAAC-OS film is described. As a formation method of the oxidesemiconductor film 342 as a CAAC-OS film, the following two kinds ofmethods can be given, for example. One of the methods is that formationof the oxide semiconductor film 342 is performed while a substrate isheated; the other method is that formation of the oxide semiconductorfilm 342 is performed in two steps, and heat treatment is performedafter each formation step of the oxide semiconductor film 342.

In the case where the oxide semiconductor film 342 is formed in one stepwhile a substrate is heated, the substrate temperature may be higherthan or equal to 100° C. and lower than or equal to 600° C., preferablyhigher than or equal to 200° C. and lower than or equal to 500° C. Whenthe substrate is heated at high temperature during formation of theoxide semiconductor film 342, the CAAC-OS film in which the proportionof a crystalline portion is higher than that of an amorphous portion canbe formed.

In the case where formation of the oxide semiconductor film 342 isperformed in two steps, a first oxide semiconductor film 342 is formedover the insulating film 324 while the substrate temperature is kept ata temperature higher than or equal to 100° C. and lower than or equal to450° C., and then heat treatment is performed at a temperature higherthan or equal to 550° C. and lower than the strain point of thesubstrate under an atmosphere of nitrogen, oxygen, a rare gas, or dryair. By the heat treatment, a crystalline region (including a plate-likecrystal) is formed in a region including a surface of the first oxidesemiconductor film 342. Next, a second oxide semiconductor film 342 isformed thicker than the first oxide semiconductor film 342. After that,heat treatment is performed again at a temperature higher than or equalto 550° C. and lower than the strain point of the substrate, so thatcrystals grow upward using, as a seed of crystal growth, the first oxidesemiconductor film 342 in which a crystalline region (including aplate-like crystal) is formed in the region including the surface. Thus,the second oxide semiconductor film 342 is entirely crystallized. Notethat the thickness of the first oxide semiconductor film 342 ispreferably greater than or equal to 1 nm and less than or equal to 10nm.

The above formation method is preferable because a short-channel effectcan be suppressed even when the thickness of the oxide semiconductorfilm 342 is approximately 5 nm.

Since the crystallinity of the crystalline portion included in theCAAC-OS film is affected by roughness of a surface where the CAAC-OSfilm is formed, as described above, the surface of the insulating film324 is preferably planarized as much as possible. The average surfaceroughness of the insulating film 324 is preferably greater than or equalto 0.1 nm and less than 0.5 nm. By planarizing the surface of theinsulating film 324, the continuity of the crystalline portion includedin the CAAC-OS film can be improved. In addition, by planarizing thesurface of the insulating film 324, the CAAC-OS film in which theproportion of a crystalline portion is higher than that of an amorphousportion can be formed.

The oxide semiconductor film 342 formed by a sputtering method containshydrogen, water, a compound having a hydroxyl group, or the like in somecases. Hydrogen, water, and the like easily form a donor level and thusserve as impurities in the oxide semiconductor. Therefore, in theformation of the oxide semiconductor film 342 by a sputtering method,the hydrogen concentration in the oxide semiconductor film 342 ispreferably reduced as much as possible.

In order to reduce the hydrogen concentration, the leakage rate of atreatment chamber of a sputtering apparatus is set to 1×10⁻¹⁰ Pa·m³/s orless in the formation of the oxide semiconductor film 342, whereby entryof impurities such as an alkali metal and hydride into the oxidesemiconductor film 342 that is being deposited by a sputtering methodcan be reduced. Further, with the use of an entrapment vacuum pump(e.g., a cryopump) as an evacuation system, counter flow of impuritiessuch as an alkali metal, a hydrogen atom, a hydrogen molecule, water, acompound having a hydroxyl group, and hydride from the evacuation systemcan be reduced.

When the purity of the target is set to 99.99% or higher, alkali metal,a hydrogen atom, a hydrogen molecule, water, a hydroxyl group, hydride,or the like mixed to the oxide semiconductor film can be reduced. Inaddition, when the target is used, the concentration of alkali metalsuch as lithium, sodium, or potassium can be reduced in the oxidesemiconductor film.

Note that it has been pointed out that an oxide semiconductor isinsensitive to impurities, there is no problem when a considerableamount of metal impurities is contained in the film, and therefore,soda-lime glass which contains a large amount of alkali metal such assodium (Na) and is inexpensive can be used (Kamiya, Nomura, and Hosono,“Carrier Transport Properties and Electronic Structures of AmorphousOxide Semiconductors: The present status”, KOTAI BUTSURI (SOLID STATEPHYSICS), 2009, Vol. 44, pp. 621-633). But such consideration is notappropriate. Alkali metal is not an element included in an oxidesemiconductor, and therefore, is an impurity. Also, alkaline earth metalis an impurity in the case where alkaline earth metal is not included inan oxide semiconductor. An alkali metal, in particular, Na becomes Na⁺when an insulating film in contact with the oxide semiconductor film isan oxide and Na diffuses into the insulating layer. In addition, in theoxide semiconductor film, Na cuts or enters a bond between metal andoxygen which are included in an oxide semiconductor. As a result, forexample, deterioration of characteristics of the transistor, such as anormally-on state of the transistor due to shift of a threshold voltagein the negative direction, or reduction in mobility, occurs. Inaddition, variation in characteristics also occurs. Such deteriorationof characteristics of the transistor and variation in characteristicsdue to the impurity remarkably appear when the hydrogen concentration inthe oxide semiconductor film is very low. Therefore, when the hydrogenconcentration in the oxide semiconductor film is less than or equal to1×10¹⁸/cm³, preferably less than or equal to 1×10¹⁷/cm³, theconcentration of the above impurity is preferably reduced. Specifically,a measurement value of a Na concentration by secondary ion massspectrometry is preferably less than or equal to 5×10¹⁶/cm³, morepreferably less than or equal to 1×10¹⁶/cm³, still more preferably lessthan or equal to 1×10¹⁵/cm³. In a similar manner, a measurement value ofa Li concentration is preferably less than or equal to 5×10¹⁵/cm³, morepreferably less than or equal to 1×10¹⁵/cm³. In a similar manner, ameasurement value of a K concentration is preferably less than or equalto 5×10¹⁵/cm³, more preferably less than or equal to 1×10¹⁵/cm³.

A highly purified rare gas (typically, argon), highly purified oxygen,or a highly purified mixed gas of oxygen and a rare gas, from whichimpurities such as hydrogen, water, a compound having a hydroxyl group,and hydride are removed, is used as appropriate as an atmosphere gassupplied to a treatment chamber of a sputtering apparatus. For example,the purity of argon is set to 9N (99.9999999%) or higher (theconcentration of H₂O is less than 0.1 ppb, and the concentration of H₂is less than 0.5 ppb), and the dew point thereof is set to −121° C. Theoxygen concentration is set to 8N (99.999999%) or higher (theconcentration of H₂O is less than 1 ppb, and the concentration of H₂ isless than 1 ppb), and the dew point thereof is set to −112° C. In thecase where a mixed gas of the rare gas and oxygen is used, the flow rateratio of oxygen is preferably high.

As one example of the film formation condition, the distance between thesubstrate and the target is 100 mm, the pressure is 0.6 Pa, thedirect-current (DC) power is 0.5 kW, and the atmosphere is an oxygenatmosphere (the proportion of the oxygen flow is 100%). Note that apulsed direct-current (DC) power source is preferable because dustgenerated in film formation can be reduced and the film thickness can bemade uniform.

In this manner, the oxide semiconductor film 342 in which the amount ofcontained hydrogen is small can be formed. Note that even when thesputtering apparatus is used, the oxide semiconductor film 342 containsmore than a little nitrogen. For example, the nitrogen concentration inthe oxide semiconductor film 342 measured by secondary ion massspectrometry (SIMS) is lower than 5×10¹⁸ atoms⁻³.

In order to reduce impurities such as moisture and hydrogen in the oxidesemiconductor film 342 (dehydration or dehydrogenation), the oxidesemiconductor film 342 is preferably subjected to heat treatment. Forexample, the oxide semiconductor film 342 is subjected to heat treatmentin a reduced-pressure atmosphere, an inert gas atmosphere of nitrogen, arare gas, or the like, an oxidation gas atmosphere, or an ultra dry airatmosphere (the moisture amount is 20 ppm (−55° C. by conversion into adew point) or less, preferably 1 ppm or less, more preferably 10 ppb orless, in the case where the measurement is performed by a dew pointmeter in a cavity ring down laser spectroscopy (CRDS) system). Note thatthe oxidation atmosphere refers to an atmosphere including an oxidationgas such as oxygen, ozone, or nitrogen oxide at 10 ppm or higher. Theinert gas atmosphere refers to an atmosphere including the oxidation gasat lower than 10 ppm and is filled with nitrogen or a rare gas.

For example, the heat treatment is performed at a temperature higherthan or equal to 150° C. and lower than the strain point of thesubstrate, preferably higher than or equal to 250° C. and lower than orequal to 450° C., more preferably higher than or equal to 300° C. andlower than or equal to 450° C. The treatment time is 3 minutes to 24hours. It is preferable that the heat treatment time be 24 hours orshorter in order not to reduce the productivity.

There is no particular limitation on a heat treatment apparatus used forthe heat treatment, and the apparatus may be provided with a device forheating an object to be processed by heat radiation or heat conductionfrom a heating element such as a resistance heating element. Forexample, an electric furnace, or a rapid thermal annealing (RTA)apparatus such as a lamp rapid thermal annealing (LRTA) apparatus or agas rapid thermal annealing (GRTA) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas.

By the heat treatment, hydrogen (water, a compound having a hydroxylgroup) can be released from the oxide semiconductor film 342. Thus,impurities in the oxide semiconductor film 342 can be reduced.

Furthermore, hydrogen that is an unstable carrier source can bedischarged from the oxide semiconductor film 342 by the heat treatment,whereby the threshold voltage of the transistor can be prevented frombeing shifted negatively. As a result, the reliability of the transistorcan be improved.

Next, a resist mask is formed through a photolithography process overthe oxide semiconductor film 342, and the oxide semiconductor film 342is etched to have a desired shape with the use of the resist mask; inthis manner, an island-shaped oxide semiconductor film 342 a is formed(see FIG. 9B). The resist mask can be formed by an ink-jet method, aprinting method, or the like as appropriate, as well as through thephotolithography process. The etching is preferably performed so that anend portion of the oxide semiconductor film 342 a has a tapered shape.The end portion of the island-shaped oxide semiconductor film 342 a istapered, whereby in the manufacturing process of the transistor 115,coverage with a film which is formed after this etching step can beimproved, and disconnection of the film can accordingly be prevented.The tapered shape can be formed by performing etching while the resistmask is made to recede.

Note that in this embodiment, the case where the heat treatment isperformed directly after the oxide semiconductor film 342 is formed isdescribed; however, the heat treatment may be performed after theisland-shaped oxide semiconductor film 342 a is obtained.

Next, after a conductive film is formed over the oxide semiconductorfilm 342 a and the like, a resist mask is formed through aphotolithography process over the conductive film and the conductivefilm is etched to have a desired shape with the use of the resist mask;in this manner, a source or drain electrode 344 a and a source or drainelectrode 344 b are formed (see FIG. 9C).

The conductive film is to be a source electrode and a drain electrodelater, and can be formed using a metal material such as aluminum,chromium, copper, titanium, tantalum, molybdenum, or tungsten.Alternatively, the conductive film can be formed using an alloycontaining any of the above metal materials as a component, or the like.Moreover, one or more materials selected from manganese, magnesium,zirconium, beryllium, neodymium, and scandium may be used.

The conductive film may have a single-layer structure or a stackedstructure of two or more layers. For example, the conductive film canhave a single-layer structure of a titanium film or a titanium nitridefilm, a single-layer structure of an aluminum film containing silicon, atwo-layer structure in which a titanium film is stacked over an aluminumfilm, a two-layer structure in which a titanium film is stacked over atitanium nitride film, or a three-layer structure in which a titaniumfilm, an aluminum film, and a titanium film are stacked. Note that whenthe conductive film has a single-layer structure of a titanium film or atitanium nitride film, there is an advantage that it can be easilyprocessed into the source or drain electrode 344 a and the source ordrain electrode 344 b having tapered shapes.

Further, as the conductive film, indium oxide, indium tin oxide (alsoreferred to as ITO), indium zinc oxide, zinc oxide, zinc oxide to whichgallium is added, graphene, or the like can be used.

The conductive film is selectively etched to form the source or drainelectrode 344 a and the source or drain electrode 344 b (see FIG. 9C).Here, the source or drain electrode 344 a functions as one of a pair ofelectrodes of the capacitor.

The conductive film is preferably etched such that the source or drainelectrode 344 a and the source or drain electrode 344 b are formed tohave tapered end portions. Here, the taper angle thereof is, forexample, preferably greater than or equal to 30° and less than or equalto 60°. When the source or drain electrode 344 a and the source or drainelectrode 344 b are formed by etching so as to have tapered endportions, coverage with the gate insulating film which is formed latercan be improved and disconnection of the gate insulating film can beprevented.

The channel length (L) of the transistor is determined by the distancebetween a lower end portion of the source or drain electrode 344 a and alower end portion of the source or drain electrode 344 b. Note that inlight exposure for forming a mask for a transistor with a channel length(L) less than 25 nm, it is preferable to use extreme ultraviolet rayswhose wavelength is as short as several nanometers to several tens ofnanometers. The resolution of light exposure with extreme ultravioletrays is high and the depth of focus is large. Accordingly, the channellength (L) of the transistor formed later can be greater than or equalto 10 nm and less than or equal to 1000 nm (1 μm), whereby the operationspeed of the circuit can be increased. Moreover, power consumption ofthe memory device can be reduced by miniaturization.

Next, a gate insulating film 346 is formed so as to cover the source ordrain electrode 344 a, the source or drain electrode 344 b, and theoxide semiconductor film 342 a (see FIG. 9D).

The gate insulating film 346 can be formed by a CVD method, a sputteringmethod, or the like. The gate insulating film 346 can be formed usingsilicon oxide, silicon nitride, silicon oxynitride, gallium oxide,aluminum oxide, tantalum oxide, or the like. Alternatively, the gateinsulating film 346 can be formed using a material with a highdielectric constant (a high-k material) such as hafnium oxide, yttriumoxide, hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate towhich nitrogen is added (HfSi_(x)O_(y)N_(z) (x>0, y>0, z>0)), or hafniumaluminate to which nitrogen is added (HfAl_(x)O_(y)N_(z) (x>0, y>0,z>0)). The gate insulating film 346 has either a single-layer structureor a stacked structure in which these materials are combined. There isno particular limitation on the thickness of the gate insulating film346; in the case where the memory device is miniaturized, the gateinsulating film 346 is preferably thin in order to ensure the operationof the transistor. For example, in the case of using silicon oxide, thethickness can be greater than or equal to 1 nm and less than or equal to100 nm, preferably greater than or equal to 10 nm and less than or equalto 50 nm.

Further, the gate insulating film 346 may be formed using an insulatingmaterial containing a Group 13 element and oxygen. Many of oxidesemiconductor materials contain a Group 13 element, and an insulatingmaterial containing a Group 13 element works well with oxidesemiconductor materials. Therefore, with the use of an insulatingmaterial containing a Group 13 element and oxygen for an insulating filmin contact with the oxide semiconductor film, an interface with theoxide semiconductor film can keep a favorable state.

Here, an insulating material containing a Group 13 element refers to aninsulating material containing one or more Group 13 elements. As theinsulating material containing a Group 13 element, gallium oxide,aluminum oxide, aluminum gallium oxide, gallium aluminum oxide, or thelike can be given as an example. Here, the amount of aluminum is largerthan that of gallium in atomic percent in aluminum gallium oxide,whereas the amount of gallium is larger than or equal to that ofaluminum in atomic percent in gallium aluminum oxide.

For example, when a material containing gallium oxide is used for thegate insulating film 346 that is in contact with the oxide semiconductorfilm 342 a containing gallium, characteristics at the interface betweenthe oxide semiconductor film and the gate insulating film can be keptfavorable. The oxide semiconductor film and an insulating filmcontaining gallium oxide are provided in contact with each other, sothat pileup of hydrogen at the interface between the oxide semiconductorfilm and the insulating film can be reduced. Note that a similar effectcan be obtained in the case where an element in the same group as aconstituent element of the oxide semiconductor is used in an insulatingfilm. For example, it is effective to form an insulating film with theuse of a material containing aluminum oxide. Note that aluminum oxidehas a property of not easily transmitting water. Thus, it is preferableto use a material containing aluminum oxide in terms of preventing entryof water into the oxide semiconductor film.

By the heat treatment performed on the oxide semiconductor film 342 (orthe oxide semiconductor film 342 a), oxygen in the oxide semiconductorfilm 342 is released together with hydrogen. When oxygen is releasedfrom the oxide semiconductor film 342, oxygen deficiency is causedtherein. Part of the oxygen deficiency becomes a donor, which leads togeneration of carriers in the oxide semiconductor film 342. As a result,characteristics of the transistor might be affected.

Therefore, an insulating film from which oxygen is discharged by heattreatment is preferably used as the gate insulating film 346 in contactwith the oxide semiconductor film 342 a.

In this specification and the like, the expression “oxygen is dischargedby heat treatment” means that the amount of discharged oxygen (orreleased oxygen) which is converted into oxygen atoms is greater than orequal to 1.0×10¹⁸ cm⁻³, preferably greater than or equal to 3.0×10²⁰cm⁻³, in thermal desorption spectroscopy (TDS) analysis. In contrast,the expression “oxygen is not discharged by heat treatment” means thatthe amount of discharged oxygen (or released oxygen) which is convertedinto oxygen atoms is less than 1.0×10¹⁸ cm⁻³ in TDS analysis.

A method for quantifying the amount of released oxygen which isconverted into oxygen atoms, with the use of TDS analysis is describedbelow.

The amount of discharged gas in TDS analysis is proportional to theintegral value of ion intensity. Therefore, the amount of discharged gascan be calculated from the ratio between the integral value of ionintensity of an insulating film and the reference value of a standardsample. The reference value of a standard sample refers to, in a samplecontaining an atom at a predetermined density, the ratio of the densityof the atom to the integral value of ion intensity corresponding to theatom.

For example, the number of the discharged oxygen molecules (N_(O2)) froman insulating film can be found according to the following formula (3)with the TDS analysis results of a silicon wafer containing hydrogen ata predetermined density which is the standard sample and the TDSanalysis results of the insulating film. Here, all gases having a massnumber of 32 which are obtained in the TDS analysis are assumed tooriginate from an oxygen molecule. Note that CH₃OH, which is given as agas having a mass number of 32, is not taken into consideration on theassumption that it is unlikely to be present. Further, an oxygenmolecule including an oxygen atom having a mass number of 17 or 18 whichis an isotope of an oxygen atom is not taken into consideration eitherbecause the proportion of such a molecule in the natural world isminimal.N _(O2) =N _(H2) /S _(H2) ×S _(O2)×α  (3)

In the formula, N_(H2) is the value obtained by conversion of the numberof hydrogen molecules desorbed from the standard sample into densities,and S_(H2) is the integral value of ion intensity when the standardsample is subjected to TDS analysis. Here, the reference value of thestandard sample is set to N_(H2)/S_(H2). Further, S_(O2) is the integralvalue of ion intensity when the insulating film is subjected to TDSanalysis, and a is a coefficient affecting the ion intensity in the TDSanalysis. Japanese Published Patent Application No. H6-275697 can bereferred to for details of the above formula. Note that the above valueof the amount of discharged oxygen is obtained by measurement with athermal desorption spectrometer produced by ESCO Ltd., EMD-WA1000S/Wusing a silicon wafer containing hydrogen atoms at 1×10¹⁶ cm⁻³ as thestandard sample.

Further, in the TDS analysis, part of oxygen is detected as an oxygenatom. The ratio between oxygen molecules and oxygen atoms can becalculated from the ionization rate of the oxygen molecules. Note that,since the above a includes the ionization rate of the oxygen molecules,the number of the discharged oxygen atoms can also be estimated throughthe evaluation of the number of the discharged oxygen molecules.

Note that N_(O2) is the number of the discharged oxygen molecules. Inthe insulating film, the amount of discharged oxygen when converted intooxygen atoms is twice the number of the discharged oxygen molecules.

As an example of a film from which oxygen is discharged by heattreatment, a film of oxygen-excess silicon oxide (SiO_(x) (x>2)) isgiven. In the oxygen-excess silicon oxide (SiO_(x) (x>2)), the number ofoxygen atoms per unit volume is more than twice the number of siliconatoms per unit volume. The number of silicon atoms and the number ofoxygen atoms per unit volume are measured by Rutherford backscatteringspectrometry.

An insulating film from which oxygen is discharged by heat treatment isused as an insulating film in contact with the oxide semiconductor film342 a (for example, the insulating film 324, the gate insulating film346), and is subjected to heat treatment in any of steps after theformation of the gate insulating film 346, so that oxygen is dischargedfrom the insulating film 324 and the gate insulating film 346 to besupplied to the oxide semiconductor film 342 a. Consequently, oxygendeficiency generated in the oxide semiconductor film 342 a can becompensated for and can be reduced. Therefore, generation of carriers inthe oxide semiconductor film 342 a can be suppressed, whereby variationin electric characteristics of the transistor can be suppressed.

Next, after a conductive film is formed over the gate insulating film346, a resist mask is formed through a photolithography process over theconductive film and the conductive film is etched to have a desiredshape with the use of the resist mask, so that a gate electrode 348 aand an electrode 348 b are formed (see FIG. 9D). A conductive layerfunctions as an electrode of the capacitor. The conductive film can beformed by using a metal material such as molybdenum, titanium, tantalum,tungsten, aluminum, copper, neodymium, or scandium, or an alloy materialcontaining any of these materials as a main component. The conductivefilm can have either a single-layer structure or a stacked structure.

After the gate electrode 348 a and the electrode 348 b are formed, adopant imparting n-type conductivity is added to the oxide semiconductorfilm 342 a with the use of the gate electrode 348 a, the source or drainelectrode 344 a, and the source or drain electrode 344 b as masks; inthis manner, a pair of dopant regions 349 a and 349 b are formed (seeFIG. 10A). In the oxide semiconductor film 342 a, a region between thedopant region 349 a and the dopant region 349 b serves a channelformation region. The channel formation region in the oxidesemiconductor film 342 a overlaps with the gate electrode 348 a with thegate insulating film 346 interposed therebetween.

The addition of the dopant for forming the dopant regions 349 a and 349b can be performed by an ion implantation method. As the dopant, forexample, a rare gas such as helium, argon, or xenon, a Group 15 elementsuch as nitrogen, phosphorus, arsenic, or antimony, or the like can beused. For example, in the case where nitrogen is used as the dopant, theconcentration of nitrogen atoms in the dopant regions 349 a and 349 b ispreferably higher than or equal to 5×10¹⁹/cm³ and lower than or equal to1×10²²/cm³. The dopant regions 349 a and 349 b to which the dopantimparting n-type conductivity is added have higher conductivity than theother regions in the oxide semiconductor film 342 a. Therefore, byproviding the dopant regions 349 a and 349 b in the oxide semiconductorfilm 342 a, the resistance between the source and drain electrodes 344 aand 344 b can be decreased.

Then, an insulating film 350 and an insulating film 352 are formed overthe gate insulating film 346, the gate electrode 348 a, and theelectrode 348 b (see FIG. 10A). The insulating film 350 and theinsulating film 352 can be formed by a PVD method, a CVD method, or thelike. The insulating film 350 and the insulating film 352 can be formedusing a material containing an inorganic insulating material such assilicon oxide, silicon oxynitride, silicon nitride, hafnium oxide,gallium oxide, or aluminum oxide, or a material containing an organicmaterial such as polyimide or acrylic. Note that for the insulating film350 and the insulating film 352, a material with a low dielectricconstant or a structure with a low dielectric constant (e.g., a porousstructure) is preferably used. This is because when the insulating film350 and the insulating film 352 have a low dielectric constant,capacitance generated between wirings, electrodes, or the like can bereduced and operation at higher speed can be achieved. For example, amaterial containing an inorganic material can be used for the insulatingfilm 350 and a material containing an organic material can be used forthe insulating film 352.

An aluminum oxide film has a property of blocking hydrogen, water, andthe like. Therefore, the insulating film 350 is preferably formed usingan aluminum oxide film in order to prevent hydrogen, water, and the likefrom entering the oxide semiconductor film 342 a from the outside of thememory device. Further, an aluminum oxide film also has a property ofblocking oxygen, so that outward diffusion of oxygen contained in theoxide semiconductor film 342 a can be suppressed. The use of an aluminumoxide film for the insulating film 350 not only can prevent hydrogen,water, and the like from entering the oxide semiconductor film 342 a butalso can suppress outward diffusion of oxygen contained in the oxidesemiconductor film 342 a. Therefore, variation in electriccharacteristics of the transistor can be suppressed.

Next, an opening reaching the source or drain electrode 344 b is formedin the gate insulating film 346, the insulating film 350, and theinsulating film 352. The opening is formed by selective etching with theuse of a mask or the like. After that, a conductive film is formed incontact with the source or drain electrode 344 b. Next, the conductivefilm is subjected to etching or CMP treatment to form an electrode 354(see FIG. 10B).

Next, a wiring 356 is formed so as to cover the insulating film 352 andbe in contact with the electrode 354 (see FIG. 10B). The wiring 356 isformed in such a manner that a conductive film is formed by a PVD methodor a CVD method and then the conductive film is processed. For theconductive film, a metal material such as aluminum, chromium, copper,tantalum, titanium, molybdenum, or tungsten, an alloy containing any ofthese metal materials as a component, or the like can be used. Any ofmanganese, magnesium, zirconium, beryllium, neodymium, and scandium, ora material including any of these in combination may be used.

Further, the wiring 356 may be formed without formation of the electrode354. For example, it is possible to employ a method in which a thintitanium film is formed in a region including the opening of theinsulating film 350 by a PVD method and then an aluminum film is formedso as to be embedded in the opening. Here, the titanium film formed by aPVD method has a function of reducing an oxide film (e.g., a naturaloxide film) formed on a surface where the titanium film is formed, todecrease contact resistance with a lower electrode or the like (here,the source or drain electrode 344 b). In addition, hillock of thealuminum film can be prevented. A copper film may be formed by a platingmethod after the formation of the barrier film of titanium, titaniumnitride, or the like.

By the wiring 356, the lower transistor and the upper transistor can beconnected to each other (not illustrated).

Thus, the memory element in which the transistor 115 including the oxidesemiconductor film 342 a and the capacitor 116 are formed is completed(see FIG. 10B).

By the above manufacturing method, the memory device in which thetransistor including an oxide semiconductor material is formed over thetransistor including a semiconductor material other than an oxidesemiconductor can be manufactured.

By the above manufacturing method, the oxide semiconductor film 342 a inwhich the amount of impurities such as hydrogen and an alkali metal isextremely small can be obtained. The hydrogen concentration in the oxidesemiconductor film 342 a can be 5×10¹⁹ atoms/cm³ or lower, preferably5×10¹⁸ atoms/cm³ or lower, more preferably 5×10¹⁷ atoms/cm³ or lower,for example. Further, as for the concentration of impurities such as Liand Na which are alkali metals and Ca which is an alkaline earth metalin the oxide semiconductor film 342 a, specifically, a measurement valueof a Na concentration by secondary ion mass spectrometry is preferablyless than or equal to 5×10¹⁶/cm³, more preferably less than or equal to1×10¹⁶/cm³, still more preferably less than or equal to 1×10¹⁵/cm³. In asimilar manner, a measurement value of a Li concentration is preferablyless than or equal to 5×10¹⁵/cm³, more preferably less than or equal to1×10¹⁵/cm³. In a similar manner, the measurement value of a Kconcentration can be preferably less than or equal to 5×10¹⁵/cm³,preferably less than or equal to 1×10¹⁵/cm³.

When the transistor 115 (and the transistor 117) is manufactured usingthe oxide semiconductor film 342 a, a transistor whose off-state currentis extremely low can be manufactured. Specifically, the off-statecurrent density can be 100 zA/μm or lower, preferably 10 zA/μm or lower.This value of off-state current density is lower than the off-statecurrent density of a transistor in which a channel is formed in acrystalline silicon film. The use of the transistor 115 for the memorycircuit 102 and the memory circuit 103 used in the memory element 110illustrated in FIG. 1 and the memory element 160 illustrated in FIG. 4enables stored data to be held for a long time because the off-statecurrent of the transistor 115 can be extremely low as described above.

The transistor according to this embodiment has relatively highfield-effect mobility; therefore, with the use of the transistor as thetransistor 115 and the transistor 117 illustrated in FIG. 1 or FIG. 4,the memory circuit 102 and the memory circuit 103 can operate at highspeed. Accordingly, in the memory device illustrated in FIG. 1 or FIG.4, data can be transferred from the logic circuit 101 to the memorycircuit 102 and the memory circuit 103 in a short time, before supply ofpower is stopped. Further, after the supply of power is restarted, datacan be restored from the memory circuit 102 and the memory circuit 103to the logic circuit 101 in a short time.

In the memory element according to one embodiment of the presentinvention, the memory circuit 102 including the transistor 115 in whicha channel is formed in an oxide semiconductor film and the memorycircuit 103 including the transistor 117 can be formed over the logiccircuit 101 and the precharge circuit 108 each including a transistor inwhich a channel is formed in a film of a semiconductor other than anoxide semiconductor. In this manner, the transistors 115 and 117 inwhich a channel is formed in an oxide semiconductor film can be stackedover a transistor in which a channel is formed in a film including asemiconductor other than an oxide semiconductor; thus, the memoryelement can be formed three-dimensionally. Therefore, the area of atwo-dimensional plane of the memory element can be decreased.

A magnetic tunneling junction element (an MTJ element) is known as anon-volatile random access memory. The MTJ element stores data in a lowresistance state when the magnetization directions of ferromagneticfilms provided above and below an insulating film are parallel, andstores data in a high resistance state when the directions are antiparallel. Therefore, the principles of the MTJ element and the memoryelement according to one embodiment of the present invention arecompletely different from each other. Table 1 shows comparison betweenthe MTJ element and the memory element according to one embodiment ofthe present invention.

TABLE 1 Spintronics (MTJ element) OS/Si 1) Heat Curie temperatureProcess temperature at 500° C. resistance (reliability at 150° C.) 2)Driving Current driving Voltage driving method 3) Writing Changingmagnetization Turning on/off FET principle direction of ferromagneticfilm 4) Si LSI Suitable for bipolar LSI Suitable for MOS LSI (For highlyintegrated circuit, MOS LSI is preferable to bipolar LSI, which isunsuitable for high integration. Note that W becomes larger.) 5)Overhead Large Smaller than overhead of MTJ (because of high Joule heat)element by 2 to 3 or more orders of magnitude (because of utilizingcharging and discharging of parasitic capacitance) 6) NonvolatilityUtilizing spin Utilizing low off-state current 7) Cycles No limitationNo limitation capable of holding charge 8) 3D structure Difficult (atmost two layers) Easy (with a limitless number of layers) 9) Integration4 F.² to 15 F.² Depending on the number of degree (F.²) layers stackedin 3D structure (need heat resistance high enough to withstand processof forming upper OS FET) 10) Material Magnetic rare-earth element OSmaterial 11) Cost per bit High Low (might be slightly high depending onconstituent of OS (e.g., In)) 12) Resistance Low High to magnetic field

The MTJ element is disadvantageous in that its magnetic properties arelost when the temperature is the Curie temperature or higher because itcontains a magnetic material. Further, the MTJ element is driven bycurrent and thus is compatible with a silicon bipolar device. However, asilicon bipolar device is unsuitable for high integration. Furthermore,the MTJ element has a problem in that its power consumption is increasedwith the increase in memory capacity, although the MTJ element requireslow write current.

In principle, the MTJ element has low resistance to a magnetic field, sothat the magnetization direction is likely to change when the MTJelement is exposed to a high magnetic field. Moreover, it is necessaryto control magnetic fluctuation due to a nanoscale magnetic materialused for the MTJ element.

In addition, a rare earth element is used for the MTJ element; thus, itrequires special attention to incorporate a process of forming the MTJelement in a process of forming a silicon semiconductor that issensitive to metal contamination. Further, the MTJ element is expensivein terms of the material cost per bit.

On the other hand, the transistor including an oxide semiconductor,which is included in this embodiment, has an element structure and anoperation principle similar to those of a silicon MOSFET except that asemiconductor material for forming a channel is a metal oxide. Further,the transistor including an oxide semiconductor is not affected by amagnetic field, and does not cause soft errors. These facts show thatthe transistor is highly compatible with a silicon integrated circuit.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 3

In this embodiment, a transistor which includes an oxide semiconductormaterial and has a structure different from the structure in Embodiment2 will be described.

A transistor 411 illustrated in FIG. 11A includes a source or drainelectrode 414 a and a source or drain electrode 414 b which are formedover a base film 412, an oxide semiconductor film 413 which is formedover the source and drain electrodes 414 a and 414 b, a gate insulatingfilm 415 over the oxide semiconductor film 413 and the source and drainelectrodes 414 a and 414 b, a gate electrode 416 provided over the gateinsulating film 415 so as to overlap with the oxide semiconductor film413, and a protective insulating film 417 provided over the gateelectrode 416 and covering the oxide semiconductor film 413.

The transistor 411 illustrated in FIG. 11A has a top-gate structurewhere the gate electrode 416 is formed over the oxide semiconductor film413, and has a bottom-contact structure where the source and drainelectrodes 414 a and 414 b are formed below the oxide semiconductor film413. In addition, the source and drain electrodes 414 a and 414 b andthe gate electrode 416 do not overlap in the transistor 411; thus,parasitic capacitance between the gate electrode 416 and the source anddrain electrodes 414 a and 414 b can be made low, so that high-speedoperation can be realized.

The oxide semiconductor film 413 includes a pair of dopant regions 418 aand 418 b which are obtained by addition of a dopant imparting n-typeconductivity to the oxide semiconductor film 413 after formation of thegate electrode 416. Further, in the oxide semiconductor film 413, aregion with which the gate electrode 416 overlaps with the gateinsulating film 415 provided therebetween is a channel formation region419. In the oxide semiconductor film 413, the channel formation region419 is provided between the pair of dopant regions 418 a and 418 b. Theaddition of the dopant for forming the dopant regions 418 a and 418 bcan be performed by an ion implantation method. A rare gas such ashelium, argon, or xenon, nitrogen, phosphorus, arsenic, antimony, boron,or the like can be used as the dopant, for example.

For example, in the case where nitrogen is used as the dopant, theconcentration of nitrogen atoms in the dopant regions 418 a and 418 b ispreferably higher than or equal to 5×10¹⁹/cm³ and lower than or equal to1×10²²/cm³.

The dopant regions 418 a and 418 b to which the dopant imparting n-typeconductivity is added have higher conductivity than the other regions inthe oxide semiconductor film 413. Therefore, by providing the dopantregions 418 a and 418 b in the oxide semiconductor film 413, theresistance between the source and drain electrodes 414 a and 414 b canbe decreased.

In the case where an In—Ga—Zn—O-based oxide semiconductor is used forthe oxide semiconductor film 413, heat treatment is performed at atemperature higher than or equal to 300° C. and lower than or equal to600° C. after nitrogen is added. Consequently, the oxide semiconductorin the dopant regions 418 a and 418 b has a wurtzite crystal structure.When the oxide semiconductor in the dopant regions 418 a and 418 b has awurtzite crystal structure, the conductivity of the dopant regions 418 aand 418 b can be further increased and the resistance between the sourceor drain electrode 414 a and the source or drain electrode 414 b can bedecreased. Note that in order to effectively decrease the resistancebetween the source or drain electrode 414 a and the source or drainelectrode 414 b by forming the oxide semiconductor having a wurtzitecrystal structure, when nitrogen is used as a dopant, the nitrogen atomconcentration in the dopant regions 418 a and 418 b is preferably higherthan or equal to 1×10²⁰/cm³ and lower than or equal to 7 atoms %.However, even when the nitrogen atom concentration is lower than theabove range, the oxide semiconductor having a wurtzite crystal structurecan be obtained in some cases.

Further, the oxide semiconductor film 413 may be a CAAC-OS film. Whenthe oxide semiconductor film 413 is a CAAC-OS film, the conductivity ofthe oxide semiconductor film 413 can be higher than that in the case ofusing an amorphous oxide semiconductor film; therefore, the resistancebetween the source and drain electrodes 414 a and 414 b can be reduced.

By reducing the resistance between the source and drain electrodes 414 aand 414 b, high on-state current and high-speed operation can be ensuredeven when the transistor 411 is miniaturized. In addition, byminiaturization of the transistor 411, the area of a semiconductordevice including the transistor can be reduced, so that the number oftransistors per unit area can be increased.

A transistor 421 illustrated in FIG. 11B includes an oxide semiconductorfilm 423 formed over a base film 422, a source or drain electrode 424 aand a source or a drain electrode 424 b which are formed over the oxidesemiconductor film 423, a gate insulating film 425 over the oxidesemiconductor film 423 and the source and drain electrodes 424 a and 424b, a gate electrode 426 provided over the gate insulating film 425 so asto overlap with the oxide semiconductor film 423, and a protectiveinsulating film 427 provided over the gate electrode 426 and coveringthe oxide semiconductor film 423. The transistor 421 further includessidewalls 430 a and 430 b provided on side surfaces of the gateelectrode 426 and formed using an insulating film.

The transistor 421 illustrated in FIG. 11B has a top-gate structurewhere the gate electrode 426 is formed over the oxide semiconductor film423, and has a top-contact structure where the source and drainelectrodes 424 a and 424 b are formed over the oxide semiconductor film423. In addition, similarly to the transistor 411, the source and drainelectrodes 424 a and 424 b and the gate electrode 426 do not overlap inthe transistor 421; thus, parasitic capacitance between the gateelectrode 426 and the source and drain electrodes 424 a and 424 b can bemade low, so that high-speed operation can be realized.

The oxide semiconductor film 423 includes a pair of high-concentrationdopant regions 428 a and 428 b and a pair of low-concentration dopantregions 429 a and 429 b which are obtained by addition of dopantsimparting n-type conductivity to the oxide semiconductor film 423 afterformation of the gate electrode 426. Further, in the oxide semiconductorfilm 423, a region with which the gate electrode 426 overlaps with thegate insulating film 425 provided therebetween is a channel formationregion 431. In the oxide semiconductor film 423, the pair oflow-concentration dopant regions 429 a and 429 b are provided betweenthe pair of high-concentration dopant regions 428 a and 428 b, and thechannel formation region 431 is provided between the pair oflow-concentration dopant regions 429 a and 429 b. The pair oflow-concentration dopant regions 429 a and 429 b are provided in regionswhich are included in the oxide semiconductor film 423 and overlap withthe sidewalls 430 a and 430 b with the gate insulating film 425 providedtherebetween.

Similarly to the dopant regions 418 a and 418 b included in thetransistor 411, the high-concentration dopant regions 428 a and 428 band the low-concentration dopant regions 429 a and 429 b can be formedby an ion implantation method. The description of the dopant regions 418a and 418 b can be referred to for the kind of the dopant for formingthe high-concentration dopant regions 428 a and 428 b.

For example, in the case where nitrogen is used as the dopant, theconcentration of nitrogen atoms in the high-concentration dopant regions428 a and 428 b is preferably higher than or equal to 5×10¹⁹/cm³ andlower than or equal to 1×10²²/cm³. Further, for example, in the casewhere nitrogen is used as the dopant, the concentration of nitrogenatoms in the low-concentration dopant regions 429 a and 429 b ispreferably higher than or equal to 5×10¹⁸/cm³ and lower than or equal to5×10¹⁹/cm³.

The high-concentration dopant regions 428 a and 428 b to which thedopant imparting n-type conductivity is added have higher conductivitythan the other regions in the oxide semiconductor film 423. Therefore,by providing the high-concentration dopant regions 428 a and 428 b inthe oxide semiconductor film 423, the resistance between the source anddrain electrodes 424 a and 424 b can be decreased. Further, thelow-concentration dopant regions 429 a and 429 b are provided betweenthe channel formation region 431 and the high-concentration dopantregions 428 a and 428 b, which results in a reduction in negative shiftof a threshold voltage due to a short-channel effect.

In the case where an In—Ga—Zn—O-based oxide semiconductor is used forthe oxide semiconductor film 423, heat treatment is performed at atemperature higher than or equal to 300° C. and lower than or equal to600° C. after nitrogen is added. Consequently, the oxide semiconductorin the high-concentration dopant regions 428 a and 428 b has a wurtzitecrystal structure. Further, depending on the nitrogen concentration, thelow-concentration dopant regions 429 a and 429 b also have a wurtzitecrystal structure due to the heat treatment. When the oxidesemiconductor in the high-concentration dopant regions 428 a and 428 bhas a wurtzite crystal structure, the conductivity of thehigh-concentration dopant regions 428 a and 428 b can be furtherincreased and the resistance between the source or drain electrode 424 aand the source or drain electrode 424 b can be decreased. Note that inorder to effectively decrease the resistance between the source or drainelectrode 424 a and the source or drain electrode 424 b by forming theoxide semiconductor having a wurtzite crystal structure, when nitrogenis used as a dopant, the nitrogen atom concentration in the dopantregions 428 a and 428 b is preferably higher than or equal to 1×10²⁰/cm³and lower than or equal to 7 atoms %. However, even when the nitrogenatom concentration is lower than the above range, the oxidesemiconductor having a wurtzite crystal structure can be obtained insome cases.

Further, the oxide semiconductor film 423 may be a CAAC-OS film. Whenthe oxide semiconductor film 423 is a CAAC-OS film, the conductivity ofthe oxide semiconductor film 423 can be higher than that in the case ofusing an amorphous oxide semiconductor film; therefore, the resistancebetween the source and drain electrodes 424 a and 424 b can be reduced.

By reducing the resistance between the source and drain electrodes 424 aand 424 b, high on-state current and high-speed operation can be ensuredeven when the transistor 421 is miniaturized. By miniaturization of thetransistor 421, the area of a memory cell array including the transistorcan be reduced, so that memory capacity per unit area can be increased.

A transistor 441 illustrated in FIG. 11C includes a source or drainelectrode 444 a and a source or drain electrode 444 b which are formedover a base film 442, an oxide semiconductor film 443 which is formedover the source and drain electrodes 444 a and 444 b and which serves asan active layer, a gate insulating film 445 over the oxide semiconductorfilm 443 and the source and drain electrodes 444 a and 444 b, a gateelectrode 446 provided over the gate insulating film 445 so as tooverlap with the oxide semiconductor film 443, and a protectiveinsulating film 447 provided over the gate electrode 446 and coveringthe oxide semiconductor film 443. The transistor 441 further includessidewalls 450 a and 450 b which are provided on side surfaces of thegate electrode 446 and formed using an insulating film.

The transistor 441 illustrated in FIG. 11C has a top-gate structurewhere the gate electrode 446 is formed over the oxide semiconductor film443, and has a bottom-contact structure where the source and drainelectrodes 444 a and 444 b are formed below the oxide semiconductor film443. In addition, similarly to the transistor 411, the source and drainelectrodes 444 a and 444 b and the gate electrode 446 do not overlap inthe transistor 441; thus, parasitic capacitance between the gateelectrode 446 and the source and drain electrodes 444 a and 444 b can bemade low, so that high-speed operation can be realized.

The oxide semiconductor film 443 includes a pair of high-concentrationdopant regions 448 a and 448 b and a pair of low-concentration dopantregions 449 a and 449 b which are obtained by addition of dopantsimparting n-type conductivity to the oxide semiconductor film 443 afterformation of the gate electrode 446. Further, in the oxide semiconductorfilm 443, a region with which the gate electrode 446 overlaps with thegate insulating film 445 provided therebetween is a channel formationregion 451. In the oxide semiconductor film 443, the pair oflow-concentration dopant regions 449 a and 449 b are provided betweenthe pair of high-concentration dopant regions 448 a and 448 b, and thechannel formation region 451 is provided between the pair oflow-concentration dopant regions 449 a and 449 b. The pair oflow-concentration dopant regions 449 a and 449 b are provided in regionswhich are included in the oxide semiconductor film 443 and with whichthe sidewalls 450 a and 450 b overlap with the gate insulating film 445provided therebetween.

Similarly to the dopant regions 418 a and 418 b included in thetransistor 411, the high-concentration dopant regions 448 a and 448 band the low-concentration dopant regions 449 a and 449 b can be formedby an ion implantation method. The description of the dopant regions 418a and 418 b can be referred to for the kind of the dopant for formingthe high-concentration dopant regions 448 a and 448 b.

For example, in the case where nitrogen is used as the dopant, theconcentration of nitrogen atoms in the high-concentration dopant regions448 a and 448 b is preferably higher than or equal to 5×10¹⁹/cm³ andlower than or equal to 1×10²²/cm³. Further, for example, in the casewhere nitrogen is used as the dopant, the concentration of nitrogenatoms in the low-concentration dopant regions 449 a and 449 b ispreferably higher than or equal to 5×10¹⁸/cm³ and lower than or equal to5×10¹⁹/cm³.

The high-concentration dopant regions 448 a and 448 b to which thedopant imparting n-type conductivity is added have higher conductivitythan the other regions in the oxide semiconductor film 443. Therefore,by providing the high-concentration dopant regions 448 a and 448 b inthe oxide semiconductor film 443, the resistance between the source anddrain electrodes 444 a and 444 b can be decreased. Further, thelow-concentration dopant regions 449 a and 449 b are provided betweenthe channel formation region 451 and the high-concentration dopantregions 448 a and 448 b, which results in a reduction in negative shiftof a threshold voltage due to a short-channel effect.

In the case where an In—Ga—Zn—O-based oxide semiconductor is used forthe oxide semiconductor film 443, heat treatment is performed at atemperature higher than or equal to 300° C. and lower than or equal to600° C. after nitrogen is added. Consequently, the oxide semiconductorin the high-concentration dopant regions 448 a and 448 b has a wurtzitecrystal structure. Further, depending on the nitrogen concentration, thelow-concentration dopant regions 449 a and 449 b also have a wurtzitecrystal structure due to the heat treatment. When the oxidesemiconductor in the high-concentration dopant regions 448 a and 448 bhas a wurtzite crystal structure, the conductivity of thehigh-concentration dopant regions 448 a and 448 b can be furtherincreased and the resistance between the source or drain electrode 444 aand the source or drain electrode 444 b can be decreased. Note that inorder to effectively decrease the resistance between the source or drainelectrode 444 a and the source or drain electrode 444 b by forming theoxide semiconductor having a wurtzite crystal structure, when nitrogenis used as a dopant, the nitrogen atom concentration in the dopantregions 448 a and 448 b is preferably higher than or equal to 1×10²⁰/cm³and lower than or equal to 7 atoms %. However, even when the nitrogenatom concentration is lower than the above range, the oxidesemiconductor having a wurtzite crystal structure can be obtained insome cases.

Further, the oxide semiconductor film 443 may be a CAAC-OS film. Whenthe oxide semiconductor film 443 is a CAAC-OS film, the conductivity ofthe oxide semiconductor film 443 can be higher than that in the case ofusing an amorphous oxide semiconductor film; therefore, the resistancebetween the source and drain electrodes 444 a and 444 b can be reduced.

By reducing the resistance between the source and drain electrodes 444 aand 444 b, high on-state current and high-speed operation can be ensuredeven when the transistor 441 is miniaturized. In addition, byminiaturization of the transistor 441, the area of a semiconductordevice including the transistor can be reduced, so that the number oftransistors per unit area can be increased.

Note that, as one of methods for manufacturing high-concentration dopantregions functioning as a source region and a drain region in atransistor including an oxide semiconductor by a self-aligned process, amethod is disclosed in which a surface of an oxide semiconductor film isexposed and argon plasma treatment is performed to reduce resistance ofthe region in the oxide semiconductor film which is exposed to plasma(S. Jeon et al. “180 nm Gate Length Amorphous InGaZnO Thin FilmTransistor for High Density Image Sensor Applications”, IEDM Tech. Dig.,pp. 504-507, 2010).

However, in the manufacturing method, a gate insulating film needs to bepartly removed after formation of the gate insulating film so thatportions which are to serve as the source region and the drain regionare exposed. At the time of partly removing the gate insulating film,part of an oxide semiconductor film below the gate insulating film isover-etched, so that the thicknesses of the portions which are to serveas the source region and the drain region are reduced. Consequently, theresistance of the source or drain region is increased, and defects incharacteristics of the transistor due to overetching easily occur.

In order to promote miniaturization of a transistor, it is necessary toemploy a dry etching method with high processing accuracy. However, theabove-described overetching remarkably tends to occur in the dry-etchingmethod by which the selection ratio of the gate insulating film to theoxide semiconductor film cannot be sufficiently provided.

For example, although overetching does not cause a problem when theoxide semiconductor film is sufficiently thick, it is necessary that thethickness of part of the oxide semiconductor film which serves as achannel formation region is less than or equal to 20 nm, preferably lessthan or equal to 10 nm when the channel length is less than or equal to200 nm, considering prevention of a short-channel effect. Overetching ofsuch a thin oxide semiconductor film is not preferable because theabove-described problem such as increase in the resistance of the sourceor drain region or defects in characteristics of the transistor occurs.

However, when a dopant is added to the oxide semiconductor film in thestate where the oxide semiconductor film is not exposed and a gateinsulating film remains, as described in one embodiment of the presentinvention, the overetching of the oxide semiconductor film can beprevented and excessive damage to the oxide semiconductor film can bereduced. In addition, the interface between the oxide semiconductor filmand the gate insulating film is kept clean. Accordingly, thecharacteristics and reliability of the transistor can be improved.

A base film positioned below the oxide semiconductor film or aprotective insulating film positioned above the oxide semiconductor filmis preferably formed using a material that has a high barrier propertyagainst an alkali metal, hydrogen, and oxygen. For example, as theinsulating film having a high barrier property, a silicon nitride film,a silicon nitride oxide film, an aluminum nitride film, an aluminumoxide film, an aluminum oxynitride film, an aluminum nitride oxide film,or the like can be used. As the base film and the protective insulatingfilm, a single layer or a stack of layers of the insulating film havinga high barrier property, or a stack of layers of the insulating filmhaving a high barrier property and the insulating film having a lowbarrier property may be used.

Covering the oxide semiconductor film with an insulating film having ahigh barrier property can prevent entry of impurities from the outsideand release of oxygen from the oxide semiconductor film. Therefore,reliability of the transistor can be improved.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 4

In this embodiment, an oxide semiconductor film including a crystal withc-axis alignment (also referred to as a CAAC-OS film) which has atriangular or hexagonal atomic arrangement when seen from the directionof an a-b plane, a surface, or an interface is described. In thecrystal, metal atoms are arranged in a layered manner along the c-axis,or metal atoms and oxygen atoms are arranged in a layered manner alongthe c-axis, and the direction of the a-axis or the b-axis is varied inthe a-b plane (the crystal twists around the c-axis).

An example of a crystal structure of the CAAC-OS film will be describedin detail with reference to FIGS. 12A to 12E, FIGS. 13A to 13C, FIGS.14A to 14C, and FIGS. 15A and 15B. In FIGS. 12A to 12E, FIGS. 13A to13C, FIGS. 14A to 14C, and FIGS. 15A and 15B, the vertical directioncorresponds to the c-axis direction and a plane perpendicular to thec-axis direction corresponds to the a-b plane, unless otherwisespecified. When the expressions “an upper half” and “a lower half” aresimply used, they refer to an upper half above the a-b plane and a lowerhalf below the a-b plane (an upper half and a lower half with respect tothe a-b plane). Furthermore, in FIGS. 12A to 12E, O surrounded by acircle represents a tetracoordinate O atom and O surrounded by a doublecircle represents a tricoordinate O atom.

FIG. 12A illustrates a structure including one hexacoordinate In atomand six tetracoordinate oxygen (hereinafter referred to astetracoordinate O) atoms proximate to the In atom. Here, a structureincluding one metal atom and oxygen atoms proximate thereto is referredto as a small group. The structure in FIG. 12A is actually an octahedralstructure, but is illustrated as a planar structure for simplicity. Notethat three tetracoordinate O atoms exist in each of an upper half and alower half in FIG. 12A. In the small group illustrated in FIG. 12A,charge is 0.

FIG. 12B illustrates a structure including one pentacoordinate Ga atom,three tricoordinate oxygen (hereinafter referred to as tricoordinate O)atoms proximate to the Ga atom, and two tetracoordinate O atomsproximate to the Ga atom. All the tricoordinate O atoms exist on the a-bplane. One tetracoordinate O atom exists in each of an upper half and alower half in FIG. 12B. An In atom can also have the structureillustrated in FIG. 12B because an In atom can have five ligands. In thesmall group illustrated in FIG. 12B, charge is 0.

FIG. 12C illustrates a structure including one tetracoordinate Zn atomand four tetracoordinate O atoms proximate to the Zn atom. In FIG. 12C,one tetracoordinate O atom exists in an upper half and threetetracoordinate O atoms exist in a lower half. Alternatively, threetetracoordinate O atoms may exist in the upper half and onetetracoordinate O atom may exist in the lower half in FIG. 12C. In thesmall group illustrated in FIG. 12C, charge is 0.

FIG. 12D illustrates a structure including one hexacoordinate Sn atomand six tetracoordinate O atoms proximate to the Sn atom. In FIG. 12D,three tetracoordinate O atoms exist in each of an upper half and a lowerhalf. In the small group illustrated in FIG. 12D, charge is +1.

FIG. 12E illustrates a small group including two Zn atoms. In FIG. 12E,one tetracoordinate O atom exists in each of an upper half and a lowerhalf. In the small group illustrated in FIG. 12E, charge is −1.

Here, a plurality of small groups form a medium group, and a pluralityof medium groups form a large group (also referred to as a unit cell).

Now, a rule of bonding between the small groups will be described. Thethree O atoms in the upper half with respect to the hexacoordinate Inatom in FIG. 12A each have three proximate In atoms in the downwarddirection, and the three O atoms in the lower half each have threeproximate In atoms in the upward direction. The one O atom in the upperhalf with respect to the pentacoordinate Ga atom in FIG. 12B has oneproximate Ga atom in the downward direction, and the one O atom in thelower half has one proximate Ga atom in the upward direction. The one Oatom in the upper half with respect to the tetracoordinate Zn atom inFIG. 12C has one proximate Zn atom in the downward direction, and thethree O atoms in the lower half each have three proximate Zn atoms inthe upward direction. In this manner, the number of tetracoordinate Oatoms above a metal atom is equal to the number of metal atoms proximateto and below the tetracoordinate O atoms; similarly, the number oftetracoordinate O atoms below a metal atom is equal to the number ofmetal atoms proximate to and above the tetracoordinate O atoms. Sincethe coordination number of the tetracoordinate O atom is 4, the sum ofthe number of the metal atoms proximate to and below the O atom and thenumber of the metal atoms proximate to and above the O atom is 4.Accordingly, when the sum of the number of tetracoordinate O atoms abovea metal atom and the number of tetracoordinate O atoms below anothermetal atom is 4, the two kinds of small groups including the metal atomscan be bonded. For example, in the case where the hexacoordinate metal(In or Sn) atom is bonded through three tetracoordinate O atoms in thelower half, it is bonded to the pentacoordinate metal (Ga or In) atom orthe tetracoordinate metal (Zn) atom.

A metal atom whose coordination number is 4, 5, or 6 is bonded toanother metal atom through a tetracoordinate O atom in the c-axisdirection. In addition to the above, a medium group can be formed in adifferent manner by combining a plurality of small groups so that thetotal charge of the layered structure is 0.

FIG. 13A illustrates a model of a medium group included in a layeredstructure of an In—Sn—Zn—O-based material. FIG. 13B illustrates a largegroup including three medium groups. Note that FIG. 13C illustrates anatomic arrangement in the case where the layered structure in FIG. 13Bis observed from the c-axis direction.

In FIG. 13A, for simplicity, a tricoordinate O atom is omitted and atetracoordinate O atom is illustrated by a circle; the number in thecircle shows the number of tetracoordinate O atoms. For example, threetetracoordinate O atoms existing in each of an upper half and a lowerhalf with respect to a Sn atom are denoted by circled 3. Similarly, inFIG. 13A, one tetracoordinate O atom existing in each of an upper halfand a lower half with respect to an In atom is denoted by circled 1.FIG. 13A also illustrates a Zn atom proximate to one tetracoordinate Oatom in a lower half and three tetracoordinate O atoms in an upper half,and a Zn atom proximate to one tetracoordinate O atom in an upper halfand three tetracoordinate O atoms in a lower half.

In the medium group included in the layered structure of theIn—Sn—Zn—O-based material in FIG. 13A, in the order starting from thetop, a Sn atom proximate to three tetracoordinate O atoms in each of anupper half and a lower half is bonded to an In atom proximate to onetetracoordinate O atom in each of an upper half and a lower half, the Inatom is bonded to a Zn atom proximate to three tetracoordinate O atomsin an upper half, the Zn atom is bonded to an In atom proximate to threetetracoordinate O atoms in each of an upper half and a lower halfthrough one tetracoordinate O atom in a lower half with respect to theZn atom, the In atom is bonded to a small group that includes two Znatoms and is proximate to one tetracoordinate O atom in an upper half,and the small group is bonded to a Sn atom proximate to threetetracoordinate O atoms in each of an upper half and a lower halfthrough one tetracoordinate O atom in a lower half with respect to thesmall group. A plurality of such medium groups are bonded, so that alarge group is formed.

Here, charge for one bond of a tricoordinate O atom and charge for onebond of a tetracoordinate O atom can be assumed to be −0.667 and −0.5,respectively. For example, charge of a (hexacoordinate orpentacoordinate) In atom, charge of a (tetracoordinate) Zn atom, andcharge of a (pentacoordinate or hexacoordinate) Sn atom are +3, +2, and+4, respectively. Accordingly, charge in a small group including a Snatom is +1. Therefore, charge of −1, which cancels +1, is needed to forma layered structure including a Sn atom. As a structure having charge of−1, the small group including two Zn atoms as illustrated in FIG. 12Ecan be given. For example, with one small group including two Zn atoms,charge of one small group including a Sn atom can be cancelled, so thatthe total charge of the layered structure can be 0.

When the large group illustrated in FIG. 13B is repeated, anIn—Sn—Zn—O-based crystal (In₂SnZn₃O₈) can be obtained. Note that alayered structure of the obtained In—Sn—Zn—O-based crystal can beexpressed as a composition formula, In₂SnZn₂O₇(ZnO)_(m) (m is 0 or anatural number).

The above-described rule also applies to the following oxides: anIn—Sn—Ga—Zn—O-based oxide which is an oxide of four metal elements; anIn—Ga—Zn—O-based oxide (also referred to as IGZO), an In—Al—Zn—O-basedoxide, a Sn—Ga—Zn—O-based oxide, an Al—Ga—Zn—O-based oxide, aSn—Al—Zn—O-based oxide, an In—Hf—Zn—O-based oxide, an In—La—Zn—O-basedoxide, an In—Ce—Zn—O-based oxide, an In—Pr—Zn—O-based oxide, anIn—Nd—Zn—O-based oxide, an In—Sm—Zn—O-based oxide, an In—Eu—Zn—O-basedoxide, an In—Gd—Zn—O-based oxide, an In—Tb—Zn—O-based oxide, anIn—Dy—Zn—O-based oxide, an In—Ho—Zn—O-based oxide, an In—Er—Zn—O-basedoxide, an In—Tm—Zn—O-based oxide, an In—Yb—Zn—O-based oxide, or anIn—Lu—Zn—O-based oxide, which is an oxide of three metal elements; anIn—Zn—O-based oxide, a Sn—Zn—O-based oxide, an Al—Zn—O-based oxide, aZn—Mg—O-based oxide, a Sn—Mg—O-based oxide, an In—Mg—O-based oxide, oran In—Ga—O-based oxide which is an oxide of two metal elements; and thelike.

As an example, FIG. 14A illustrates a model of a medium group includedin a layered structure of an In—Ga—Zn—O-based material.

In the medium group included in the layered structure of theIn—Ga—Zn—O-based material in FIG. 14A, in the order starting from thetop, an In atom proximate to three tetracoordinate O atoms in each of anupper half and a lower half is bonded to a Zn atom proximate to onetetracoordinate O atom in an upper half, the Zn atom is bonded to a Gaatom proximate to one tetracoordinate O atom in each of an upper halfand a lower half through three tetracoordinate O atoms in a lower halfwith respect to the Zn atom, and the Ga atom is bonded to an In atomproximate to three tetracoordinate O atoms in each of an upper half anda lower half through one tetracoordinate O atom in a lower half withrespect to the Ga atom. A plurality of such medium groups are bonded, sothat a large group is formed.

FIG. 14B illustrates a large group including three medium groups. Notethat FIG. 14C illustrates an atomic arrangement in the case where thelayered structure in FIG. 14B is observed from the c-axis direction.

Here, since charge of a (hexacoordinate or pentacoordinate) In atom,charge of a (tetracoordinate) Zn atom, and charge of a (pentacoordinate)Ga atom are +3, +2, and +3, respectively, charge of a small groupincluding any of an In atom, a Zn atom, and a Ga atom is 0. As a result,the total charge of a medium group having a combination of such smallgroups is always 0.

In order to form the layered structure of the In—Ga—Zn—O-based material,a large group can be formed using not only the medium group illustratedin FIG. 14A but also a medium group in which the arrangement of the Inatom, the Ga atom, and the Zn atom is different from that in FIG. 14A.

When the large group illustrated in FIG. 14B is repeated, anIn—Ga—Zn—O-based crystal can be obtained. Note that a layered structureof the obtained In—Ga—Zn—O-based crystal can be expressed as acomposition formula, InGaO₃ (ZnO)_(n) (n is a natural number).

In the case where n=1 (InGaZnO₄), a crystal structure illustrated inFIG. 15A can be obtained, for example. Note that in the crystalstructure in FIG. 15A, since a Ga atom and an In atom each have fiveligands as described with FIG. 12B, a structure in which Ga is replacedwith In can be obtained.

In the case where n=2 (InGaZn₂O₅), a crystal structure illustrated inFIG. 15B can be obtained, for example. Note that in the crystalstructure in FIG. 15B, since a Ga atom and an In atom each have fiveligands as described with FIG. 12B, a structure in which Ga is replacedwith In can be obtained.

As described above, a variety of crystal structures of the CAAC-OS filmcan be obtained.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 5

In this embodiment, the field-effect mobility of a transistor will bedescribed.

The actually measured field-effect mobility of an insulated gatetransistor can be lower than its original mobility because of a varietyof reasons; this phenomenon occurs not only in the case of using atransistor whose channel is formed in an oxide semiconductor film. Oneof the reasons that reduce the mobility is a defect inside asemiconductor or a defect at the interface between the semiconductor andan insulating film. When a Levinson model is used, the field-effectmobility that is based on the assumption that no defect exists insidethe semiconductor can be calculated theoretically.

Assuming that the original mobility and the measured field-effectmobility of a semiconductor are μ₀ and μ, respectively, and a potentialbarrier (such as a grain boundary) exists in the semiconductor, themeasured field-effect mobility t can be expressed as the followingformula (4).

$\begin{matrix}{\mu = {\mu_{0}{\exp( {- \frac{E}{kT}} )}}} & (4)\end{matrix}$

Here, E represents the height of the potential barrier, k represents theBoltzmann constant, and T represents the absolute temperature. When thepotential barrier is assumed to be attributed to a defect, the height Eof the potential barrier can be expressed as the following formula (5)according to the Levinson model.

$\begin{matrix}{E = {\frac{e^{2}N^{2}}{8ɛ\; n} = \frac{e^{2}N^{2}t}{8ɛ\; C_{ox}V_{g}}}} & (5)\end{matrix}$

Here, e represents the elementary charge, N represents the averagedefect density per unit area in a channel, ∈ represents the permittivityof the semiconductor, n represents the number of carriers per unit areain the channel, C_(ox) represents the capacitance per unit area, V_(g)represents the gate voltage, and t represents the thickness of thechannel. Note that in the case where the thickness of the semiconductorlayer is less than or equal to 30 nm, the thickness of the channel maybe regarded as being the same as the thickness of the semiconductorlayer. The drain current I_(d) in a linear region can be expressed asthe following formula (6).

$\begin{matrix}{I_{d} = {\frac{W_{\mu}V_{g}V_{d}C_{ox}}{L}{\exp( {- \frac{E}{kT}} )}}} & (6)\end{matrix}$

Here, L represents the channel length and W represents the channelwidth, and L and W are each 10 μm. Further, V_(d) represents the drainvoltage. When dividing both sides of the above equation by V_(g) andthen taking logarithms of both sides, the following formula (7) can beobtained.

$\begin{matrix}{{\ln( \frac{I_{d}}{V_{g}} )} = {{{\ln( \frac{W_{\mu}V_{d}C_{ox}}{L} )} - \frac{E}{kT}} = {{\ln( \frac{W_{\mu}V_{d}C_{ox}}{L} )} - \frac{e^{3}N^{2}t}{8\;{kT}\; ɛ\; C_{ox}V_{g}}}}} & (7)\end{matrix}$

The right side of the formula (7) is a function of V_(g). From theformula, it is found that the defect density N can be obtained from theslope of a line in which ln(I_(d)/V_(g)) is the ordinate and 1/V_(g) isthe abscissa. That is, the defect density can be evaluated from theI_(d)−V_(g) characteristics of the transistor. The defect density N ofan oxide semiconductor in which the ratio of indium (In), tin (Sn), andzinc (Zn) is 1:1:1 is approximately 1×10¹²/cm².

On the basis of the defect density obtained in this manner, or the like,μ₀ can be calculated to be 120 cm²/Vs from the formula 4 and the formula5. The measured mobility of an In—Sn—Zn oxide including a defect isapproximately 40 cm²/Vs. However, assuming that no defect exists insidethe semiconductor and at the interface between the semiconductor and aninsulating film, the mobility μ₀ of the oxide semiconductor is expectedto be 120 cm²/Vs.

Note that even when no defect exists inside a semiconductor, scatteringat the interface between a channel and a gate insulating layer adverselyaffects the transport property of the transistor. In other words, themobility μ₁ at a position that is distance x away from the interfacebetween the channel and the gate insulating layer can be expressed asthe following formula (8).

$\begin{matrix}{\frac{1}{\mu_{1}} = {\frac{1}{\mu_{0}} + {\frac{D}{B}{\exp( {- \frac{x}{l}} )}}}} & (8)\end{matrix}$

Here, D represents the electric field in the gate direction, and B and lare constants. Note that B and l can be obtained from actual measurementresults; according to the above measurement results, B is 4.75×10⁷ cm/sand l is 10 nm (the depth to which the influence of interface scatteringreaches). When D is increased (i.e., when the gate voltage isincreased), the second term of the formula 6 is increased andaccordingly the mobility μ₁ is decreased.

FIG. 16 shows calculation results of the mobility μ₂ of a transistorwhose channel is formed using an ideal oxide semiconductor without adefect inside the semiconductor. For the calculation, device simulationsoftware Sentaurus Device manufactured by Synopsys, Inc. was used, andthe bandgap, the electron affinity, the relative permittivity, and thethickness of the oxide semiconductor were assumed to be 2.8 eV, 4.7 eV,15, and 15 nm, respectively. These values were obtained by measurementof a thin film that was formed by a sputtering method.

Further, the work functions of a gate, a source, and a drain wereassumed to be 5.5 eV, 4.6 eV, and 4.6 eV, respectively. The thickness ofa gate insulating layer was assumed to be 100 nm, and the relativepermittivity thereof was assumed to be 4.1. The channel length and thechannel width were each assumed to be 10 μm, and the drain voltage V_(d)was assumed to be 0.1 V.

As shown in FIG. 16, the mobility has a peak of 100 cm²/Vs or more at agate voltage that is a little over 1 V, and is decreased as the gatevoltage becomes higher because the influence of interface scattering isincreased. Note that in order to reduce interface scattering, it isdesirable that a surface of the semiconductor layer be flat at theatomic level (atomic layer flatness).

Calculation results of characteristics of minute transistors formedusing an oxide semiconductor having such a mobility are shown in FIGS.17A to 17C, FIGS. 18A to 18C, and FIGS. 19A to 19C. FIGS. 20A and 20Billustrate cross-sectional structures of the transistors used for thecalculation. The transistors illustrated in FIGS. 20A and 20B eachinclude a semiconductor region 1103 a and a semiconductor region 1103 cthat have n⁺-type conductivity in an oxide semiconductor layer. Theresistivity of the semiconductor regions 1103 a and 1103 c is 2×10⁻³Ωcm.

The transistor in FIG. 20A is formed over a base insulating layer 1101and an embedded insulator 1102 that is embedded in the base insulatinglayer 1101 and formed of aluminum oxide. The transistor includes thesemiconductor region 1103 a, the semiconductor region 1103 c, anintrinsic semiconductor region 1103 b that is placed between thesemiconductor regions 1103 a and 1103 c and serves as a channelformation region, and a gate electrode 1105. The width of the gateelectrode 1105 is 33 nm.

A gate insulating film 1104 is formed between the gate electrode 1105and the semiconductor region 1103 b. A sidewall insulating layer 1106 aand a sidewall insulating layer 1106 b are formed on both side surfacesof the gate electrode 1105, and an insulating layer 1107 is formed overthe gate electrode 1105 so as to prevent a short circuit between thegate electrode 1105 and another wiring. The sidewall insulating layerhas a width of 5 nm. A source or drain electrode 1108 a and a source ordrain electrode 1108 b are provided in contact with the semiconductorregion 1103 a and the semiconductor region 1103 c, respectively. Notethat the channel width of this transistor is 40 nm.

The transistor in FIG. 20B is the same as the transistor in FIG. 20A inthat it is formed over the base insulating layer 1101 and the embeddedinsulator 1102 formed of aluminum oxide and that it includes thesemiconductor region 1103 a, the semiconductor region 1103 c, theintrinsic semiconductor region 1103 b provided therebetween, the gateelectrode 1105 having a width of 33 nm, the gate insulating film 1104,the sidewall insulating layer 1106 a, the sidewall insulating layer 1106b, the insulating layer 1107, the source or drain electrode 1108 a, andthe source or drain electrode 1108 b.

The difference between the transistor in FIG. 20A and the transistor inFIG. 20B is the conductivity type of semiconductor regions under thesidewall insulating layers 1106 a and 1106 b. In the transistor in FIG.20A, the semiconductor regions under the sidewall insulating layer 1106a and the sidewall insulating layer 1106 b are part of the semiconductorregion 1103 a having n⁺-type conductivity and part of the semiconductorregion 1103 c having n⁺-type conductivity, whereas in the transistor inFIG. 20B, the semiconductor regions under the sidewall insulating layer1106 a and the sidewall insulating layer 1106 b are part of theintrinsic semiconductor region 1103 b. In other words, in thesemiconductor layer of FIG. 20B, a region having a width of L_(off)which overlaps with neither the semiconductor region 1103 a (thesemiconductor region 1103 c) nor the gate electrode 1105 is provided.This region is called an offset region, and the width L_(off) is calledan offset length. As is seen from the drawing, the offset length isequal to the width of the sidewall insulating layer 1106 a (the sidewallinsulating layer 1106 b).

The other parameters used in calculation are as described above. For thecalculation, device simulation software Sentaurus Device manufactured bySynopsys, Inc. was used. FIGS. 17A to 17C show the gate voltage (V_(g):a potential difference between the gate and the source) dependence ofthe drain current (I_(d), a solid line) and the mobility (μ, a dottedline) of the transistor having the structure illustrated in FIG. 20A.The drain current I_(d) is obtained by calculation under the assumptionthat the drain voltage (a potential difference between the drain and thesource) is +1 V, and the mobility μ is obtained by calculation under theassumption that the drain voltage is +0.1 V.

FIG. 17A shows the gate voltage dependence of the transistor in the casewhere the thickness of the gate insulating film is 15 nm, FIG. 17B showsthat of the transistor in the case where the thickness of the gateinsulating film is 10 nm, and FIG. 17C shows that of the transistor inthe case where the thickness of the gate insulating film is 5 nm. As thegate insulating film is thinner, the drain current I_(d) in an off state(the off-state current) in particular is significantly decreased. Incontrast, there is no noticeable change in peak value of the mobility μand the drain current I_(d) in an on state (the on-state current). Thegraphs show that the drain current exceeds 10 μA, which is required in amemory element and the like, at a gate voltage of around 1 V.

FIGS. 18A to 18C show the gate voltage V_(g) dependence of the draincurrent I_(d) (a solid line) and the mobility μ (a dotted line) of thetransistor having the structure in FIG. 20B and an offset length L_(off)of 5 nm. The drain current I_(d) is obtained by calculation under theassumption that the drain voltage is +1 V, and the mobility μ isobtained by calculation under the assumption that the drain voltage is+0.1 V. FIG. 18A shows the gate voltage dependence of the transistor inthe case where the thickness of the gate insulating film is 15 nm, FIG.18B shows that of the transistor in the case where the thickness of thegate insulating film is 10 nm, and FIG. 18C shows that of the transistorin the case where the thickness of the gate insulating film is 5 nm.

FIGS. 19A to 19C show the gate voltage dependence of the drain currentI_(d) (a solid line) and the mobility μ (a dotted line) of thetransistor having the structure in FIG. 20B and an offset length L_(off)of 15 nm. The drain current I_(d) is obtained by calculation under theassumption that the drain voltage is +1 V, and the mobility μ isobtained by calculation under the assumption that the drain voltage is+0.1 V. FIG. 19A shows the gate voltage dependence of the transistor inthe case where the thickness of the gate insulating film is 15 nm, FIG.19B shows that of the transistor in the case where the thickness of thegate insulating film is 10 nm, and FIG. 19C shows that of the transistorin the case where the thickness of the gate insulating film is 5 nm.

In either of the structures, as the gate insulating film is thinner, theoff-state current is significantly decreased, whereas no noticeablechange arises in the peak value of the mobility μ and the on-statecurrent.

Note that the peak of the mobility t is approximately 80 cm²/Vs in FIGS.17A to 17C, approximately 60 cm²/Vs in FIGS. 18A to 18C, andapproximately 40 cm²/Vs in FIGS. 19A to 19C; thus, the peak of themobility t is decreased as the offset length L_(off) is increased.Further, the same applies to the off-state current. The on-state currentis also decreased as the offset length L_(off) is increased; however,the decrease in the on-state current is much more gradual than thedecrease in the off-state current. Further, the graphs show that ineither of the structures, the drain current exceeds 10 μA, which isrequired in a memory element and the like, at a gate voltage of around 1V.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 6

In this embodiment, electric characteristics and off-state current of atransistor including an oxide semiconductor, which can be used in amemory device according to one embodiment of the present invention, willbe described.

FIGS. 21A and 21B are a top view and a cross-sectional view of each oftransistors (Sample 1 and Sample 2). FIG. 21A is a top view of eachtransistor. FIG. 21B is a cross-sectional view along dashed-dotted lineA-B in FIG. 21A.

The transistor shown in FIG. 21B includes a substrate 600; a baseinsulating film 602 provided over the substrate 600; an oxidesemiconductor film 606 provided over the base insulating film 602; apair of electrodes 614 in contact with the oxide semiconductor film 606;a gate insulating film 608 provided over the oxide semiconductor film606 and the pair of electrodes 614; a gate electrode 610 provided tooverlap with the oxide semiconductor film 606 with the gate insulatingfilm 608 provided therebetween; an interlayer insulating film 616provided to cover the gate insulating film 608 and the gate electrode610; wirings 618 connected to the pair of electrodes 614 throughopenings formed in the gate insulating film 608 and the interlayerinsulating film 616; and a protective film 620 provided to cover theinterlayer insulating film 616 and the wirings 618.

A glass substrate can be used as the substrate 600. A silicon oxide filmcan be used as the base insulating film 602. An In—Sn—Zn—O film can beused as the oxide semiconductor film 606. A tungsten film can be used asthe pair of electrodes 614. A silicon oxide film can be used as the gateinsulating film 608. A stacked-layer structure of a tantalum nitridefilm and a tungsten film can be used for the gate electrode 610. Astacked-layer structure of a silicon oxynitride film and a polyimidefilm can be used for the interlayer insulating film 616. A stacked-layerstructure in which a titanium film, an aluminum film, and a titaniumfilm are stacked in this order can be used for each of the wirings 618.A polyimide film can be used as the protective film 620.

In the transistor having the structure illustrated in FIG. 21A, thewidth of a portion where the gate electrode 610 overlaps with theelectrode 614 is referred to as Lov. In addition, the width of a portionof the electrode 614 which does not overlap with the oxide semiconductorfilm 606 is referred to as dW.

A method for forming the transistors (Samples 1 and 2) having thestructure illustrated in FIG. 21B is described below.

First, plasma treatment is performed on a surface of the substrate 600in an argon atmosphere. The plasma treatment is carried out with asputtering apparatus by applying a bias power of 200 W (RF) to thesubstrate 600 side for 3 minutes.

Subsequently, without breaking the vacuum, a silicon oxide film as thebase insulating film 602 is formed to have a thickness of 300 nm.

The silicon oxide film is formed with a sputtering apparatus with apower of 1500 W (RF) in an oxygen atmosphere. A quartz target is used asa target. The substrate heating temperature in the film deposition isset at 100° C.

Next, a surface of the base insulating film 602 is processed by CMP tobe planarized such that R_(a) is about 0.2 nm.

Next, over the planarized base insulating film 602, an In—Sn—Zn—O filmas the oxide semiconductor film is formed to have a thickness of 15 nm.

The In—Sn—Zn—O film is formed with a sputtering apparatus with a powerof 100 W (DC) in a mixed atmosphere of argon:oxygen=2:3 [volume ratio].An In—Sn—Zn—O target of In:Sn:Zn=1:1:1 [atomic ratio] is used as atarget. The substrate heating temperature in the film deposition is setat 200° C.

Next, heat treatment is performed only on Sample 2 at 650° C. As theheat treatment, heat treatment in a nitrogen atmosphere is firstperformed for 1 hour and then heat treatment in an oxygen atmosphere isperformed for 1 hour while keeping the temperature.

Then, the oxide semiconductor film is processed by a photolithographyprocess, whereby the oxide semiconductor film 606 is formed.

Next, the tungsten film is formed over the oxide semiconductor film 606to have a thickness of 50 nm.

The tungsten film is formed with a sputtering apparatus with a power of1000 W (DC) in an argon atmosphere. The substrate heating temperature inthe film deposition is set at 200° C.

Then, the tungsten film is processed by a photolithography process,whereby the pair of electrodes 614 are formed.

Next, a silicon oxide film as the gate insulating film 608 is formed tohave a thickness of 100 nm. The relative permittivity of the siliconoxide film is set at 3.8.

The silicon oxide film as the gate insulating film 608 can be formed ina similar manner to that of the base insulating film 602.

Next, over the gate insulating film 608, a tantalum nitride film and atungsten film are formed in this order to have thicknesses of 15 nm and135 nm, respectively.

The tantalum nitride film is formed with a sputtering apparatus with apower of 1000 W (DC) in a mixed atmosphere of argon:nitrogen=5:1.Substrate heating is not performed in the film deposition.

The tungsten film is formed with a sputtering apparatus with a power of4000 W (DC) in an argon atmosphere. The substrate heating temperature inthe film deposition is set at 200° C.

Then, the tantalum nitride film and the tungsten film are processed by aphotolithography process, whereby the gate electrode 610 is formed.

Next, a silicon oxynitride film as part of the interlayer insulatingfilm 616 is formed over the gate insulating film 608 and the gateelectrode 610 to have a thickness of 300 nm.

The silicon oxynitride film as part of the interlayer insulating film616 is formed with a PCVD apparatus with a power of 35 W (RF) in a mixedatmosphere of monosilane:nitrous oxide=1:200. The substrate heatingtemperature in the film deposition is set at 325° C.

Then, the silicon oxynitride film as part of the interlayer insulatingfilm 616 is processed by a photolithography process.

Next, photosensitive polyimide as part of the interlayer insulating film616 is deposited to have a thickness of 1500 nm.

Next, the photosensitive polyimide as part of the interlayer insulatingfilm 616 is exposed to light using a photomask which is used in thephotolithography process on the silicon oxynitride film as part of theinterlayer insulating film 616, and developed, and then subjected toheat treatment for hardening the photosensitive polyimide film. In thismanner, the interlayer insulating film 616 is formed of the siliconoxynitride film and the photosensitive polyimide film. The heattreatment is performed in a nitrogen atmosphere at 300° C.

Next, a titanium film, an aluminum film, and a titanium film are formedin this order to have thicknesses of 50 nm, 100 nm, and 5 nm,respectively.

The two titanium films are formed with a sputtering apparatus with apower of 1000 W (DC) in an argon atmosphere. Note that heating is notperformed on the substrate during deposition.

The aluminum film is formed with a sputtering apparatus with a power of1000 W (DC) in an argon atmosphere. Note that heating is not performedon the substrate during deposition.

Then, the titanium film, the aluminum film, and the titanium film areprocessed by a photolithography process, whereby the wirings 618 areformed.

Next, a photosensitive polyimide film as the protective film 620 isformed to have a thickness of 1500 nm.

Next, the photosensitive polyimide film is exposed to light with the useof a photomask which is used in the photolithography process on thewirings 618, and developed, so that openings at which the wirings 618are exposed are formed in the protective film 620.

Next, heat treatment for hardening the photosensitive polyimide film isperformed thereon. The heat treatment is performed in a similar mannerto that of the heat treatment performed on the photosensitive polyimidefilm as the interlayer insulating film 616.

Through the above process, the transistors (Samples 1 and 2) having thestructure illustrated in FIG. 21B can be formed.

Next, evaluation results of electric characteristics of the transistors(Samples 1 and 2) having the structure illustrated in FIG. 21B aredescribed.

Here, V_(gs)−I_(ds) characteristics of the transistors (Samples 1 and 2)having the structure illustrated in FIG. 21B were measured; the resultsof Sample 1 are shown in FIG. 22A and the results of Sample 2 are shownin FIG. 22B. Each transistor used for the measurement has a channellength L of 3 μm, a channel width W of 10 μm, Lov of 3 μm per side (6 μmin total), and dW of 3 μm per side (6 μm in total). Note that V_(ds) isset at 10 V.

Comparing Samples 1 and 2, it is found that from the results of Sample2, the field-effect mobility of the transistor is increased byperforming heat treatment after formation of the oxide semiconductorfilm. The reason for this is deemed that the impurity concentration inthe oxide semiconductor film is reduced by the heat treatment;accordingly, it is understood that the impurity concentration in theoxide semiconductor film is reduced by heat treatment performed afterthe oxide semiconductor film is formed, whereby the field-effectmobility of the transistor can be increased.

Next, evaluation results of the off-state current (per micrometer of achannel width) of the transistor which can be applied to a memory deviceaccording to one embodiment of the present invention are described.

The transistor used in the measurement has a channel length L of 3 μm, achannel width W of 10 μm, Lov of 2 μm, and dW of 0 μm.

FIG. 23 shows a relation between the off-state current of the transistorand the inverse of the substrate temperature (absolute temperature) atmeasurement. For simplicity, a value (1000/T) obtained by multiplyingthe inverse of the substrate temperature at measurement by 1000 isindicated by the horizontal axis.

A method for measuring the off-state current of the transistor is simplydescribed below. Note that a transistor which is an object to bemeasured is called a first transistor for convenience.

A drain of the first transistor is connected to a floating gate FG, andthe floating gate FG is connected to a gate of a second transistor.

First, the first transistor is turned off, and charge is supplied to thefloating gate FG, where a certain drain voltage is applied to the secondtransistor.

Consequently, the charge at the floating gate FG gradually leaks throughthe first transistor to change the source potential of the secondtransistor. The amount of charge leaked from the first transistor can beestimated from that amount of change of the source potential in relationto time, whereby the off-state current can be measured.

FIG. 23 shows that the off-state current of the transistor is 2×10⁻²¹A/μm (2 zA/μm) when a substrate temperature in measurement is 85° C. Theproportional relation between the logarithm of the off-state current andthe inverse of the substrate temperature suggests that the off-statecurrent at room temperature (27° C.) is smaller than 1×10⁻²² A/μm (0.1zA/μm).

As described above, it is found that the off-state current of thetransistor according to this embodiment is extremely small.

When the transistor of this embodiment is used as each of thetransistors 115 and 117 illustrated in FIG. 1 or FIG. 4, potentials heldin the nodes M and N can be held for a long time. In the memory deviceillustrated in FIG. 1 or FIG. 4, the potentials held in the nodes O andP of the logic circuit 101 can be held in the nodes M and N after thesupply of power is stopped.

The transistor according to this embodiment has relatively highfield-effect mobility; therefore, with the use of the transistor as thetransistor 115 and the transistor 117 illustrated in FIG. 1 or FIG. 4,the memory circuit 102 and the memory circuit 103 can operate at highspeed. Accordingly, in the memory device illustrated in FIG. 1 or FIG.4, data can be transferred from the logic circuit 101 to the memorycircuit 102 and the memory circuit 103 in a short time, before supply ofpower is stopped. Further, after the supply of power is restarted, datacan be restored from the memory circuit 102 and the memory circuit 103to the logic circuit 101 in a short time.

This embodiment can be implemented in appropriate combination with anyof the above embodiments.

Embodiment 7

In this embodiment, a structure of a signal processing circuit includingany of the memory devices described in the above embodiments will bedescribed.

FIG. 24 illustrates an example of a signal processing circuit accordingto one embodiment of the present invention. The signal processingcircuit at least includes one or a plurality of arithmetic circuits andone or a plurality of memory devices. Specifically, a signal processingcircuit 500 illustrated in FIG. 24 includes an arithmetic circuit 501,an arithmetic circuit 502, a memory device 503, a memory device 504, amemory device 505, a control device 506, a power supply control circuit507, and a memory device 508.

The arithmetic circuits 501 and 502 each include, as well as a logiccircuit which carries out simple logic arithmetic processing, an adder,a multiplier, and various arithmetic circuits. The memory device 503functions as a register for temporarily holding data when the arithmeticprocessing is carried out in the arithmetic circuit 501. The memorydevice 504 functions as a register for temporarily holding data when thearithmetic processing is carried out in the arithmetic circuit 502.

In addition, the memory device 505 can be used as a main memory and canstore a program executed by the control device 506 as data or can storedata from the arithmetic circuit 501 and the arithmetic circuit 502.

The control device 506 is a circuit which collectively controlsoperations of the arithmetic circuit 501, the arithmetic circuit 502,the memory device 503, the memory device 504, the memory device 505, andthe memory device 508 included in the signal processing circuit 500.Note that in FIG. 24, a structure in which the control device 506 isprovided in the signal processing circuit 500 as a part thereof isillustrated, but the control device 506 may be provided outside thesignal processing circuit 500.

In addition, as well as the supply of the power supply voltage to thememory device, the supply of the power supply voltage to the controlcircuit or the arithmetic circuit which transmits/receives data to/fromthe memory device may be stopped. For example, when the arithmeticcircuit 501 and the memory device 503 are not operated, the supply ofthe power supply voltage to the arithmetic circuit 501 and the memorydevice 503 may be stopped.

In addition, the power supply control circuit 507 controls the level ofthe power supply voltage which is supplied to the arithmetic circuit501, the arithmetic circuit 502, the memory device 503, the memorydevice 504, the memory device 505, the control device 506, and thememory device 508 included in the signal processing circuit 500.Further, in the case where the supply of the power supply voltage isstopped, a switching element for stopping the supply of the power supplyvoltage may be provided for the power supply control circuit 507, or foreach of the arithmetic circuit 501, the arithmetic circuit 502, thememory device 503, the memory device 504, the memory device 505, thecontrol device 506, and the memory device 508. In the latter case, thepower supply control circuit 507 is not necessarily provided in thesignal processing circuit according to one embodiment of the presentinvention.

The memory device 508 which functions as a cache memory is preferablyprovided between the memory device 505 that is a main memory and thecontrol device 506. By providing the cache memory, access to thelow-speed main memory can be reduced and the speed of the signalprocessing such as arithmetic processing can be higher.

When a memory device according to one embodiment of the presentinvention is used as each of the memory devices 503, 504, and 508, dataof the memory device can be held even when the supply of the powersupply voltage is stopped for a short time. Further, data held in thememory device is not necessarily transferred to an external non-volatilememory device; therefore, the supply of the power supply voltage can bestopped in a short time. Furthermore, after the supply of power supplyvoltage is restarted, data held in the memory device can be restored toa state before the supply of the power supply voltage stopped, in ashort time. The use of the above-described memory device 503, memorydevice 504, and memory device 508 for the signal processing circuit 500can reduce power consumption in the case where the supply of power isstopped for a short time.

This embodiment can be implemented in appropriate combination with anyof the above-described embodiments.

Embodiment 8

A memory device or a signal processing circuit according to oneembodiment of the present invention can be applied to a variety ofelectronic devices (including game machines). Examples of electronicdevices are a television set (also referred to as a television or atelevision receiver), a monitor of a computer or the like, a camera suchas a digital camera or a digital video camera, a digital photo frame, amobile phone handset (also referred to as a mobile phone or a mobilephone device), a portable game machine, a portable information terminal,an audio reproducing device, a large-sized game machine such as apachinko machine, and the like. Examples of an electronic deviceincluding the memory device or the signal processing circuit describedin any of the above embodiments are described below.

FIG. 25A illustrates a laptop personal computer, which includes a mainbody 911, a housing 912, a display portion 913, a keyboard 914, and thelike. The housing 912 includes the memory device or the signalprocessing circuit according to one embodiment of the present invention.Therefore, in the case where supply of power is stopped for a shorttime, power consumption of the laptop personal computer can be reduced.

FIG. 25B is a personal digital assistant (PDA), which includes a mainbody 921 provided with a display portion 923, an external interface 925,operation buttons 924, and the like. A stylus 922 is included as anaccessory for operation. The main body 921 includes the memory device orthe signal processing circuit according to one embodiment of the presentinvention. Therefore, in the case where supply of power is stopped for ashort time, power consumption of the personal digital assistant can bereduced.

FIG. 25C illustrates an example of an e-book reader. For example, ane-book reader 930 includes two housings, a housing 931 and a housing932. The housing 931 and the housing 932 are combined with a hinge 935so that the e-book reader 930 can be opened and closed with the hinge935 as an axis. With such a structure, the e-book reader 930 can operatelike a paper book.

A display portion 933 and a display portion 934 are incorporated in thehousing 931 and the housing 932, respectively. One screen image ordifferent screen images may be displayed on the display portion 933 andthe display portion 934. In the structure where different screen imagesare displayed on the display portion 933 and the display portion 934,for example, text can be displayed on the right display portion (thedisplay portion 934 in FIG. 25C) and images can be displayed on the leftdisplay portion (the display portion 933 in FIG. 25C). At least one ofthe housings 931 and 932 includes the memory device or the signalprocessing circuit according to one embodiment of the present invention.Therefore, in the case where supply of power is stopped for a shorttime, power consumption of the e-book reader can be reduced.

FIG. 25C illustrates an example in which the housing 932 is providedwith an operation portion and the like. For example, the housing 932 isprovided with a power switch 936, an operation key 937, a speaker 938,and the like. With the operation key 937, pages can be turned. Akeyboard, a pointing device, or the like may also be provided on thesame plane of the housing, as the display portion. Furthermore, anexternal connection terminal (an earphone terminal, a USB terminal, orthe like), a recording medium insertion portion, and the like may beprovided on the back surface or the side surface of the housing. Thee-book reader 930 may have a function of an electronic dictionary.

The e-book reader 930 may have a configuration capable of wirelesslytransmitting and receiving data. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an electronicbook server.

FIG. 25D illustrates a mobile phone, which includes two housings, ahousing 940 and a housing 941. The housing 941 is provided with adisplay panel 942, a speaker 943, a microphone 944, a pointing device946, a camera lens 947, an external connection terminal 948, and thelike. The housing 940 is provided with a solar cell 949 for charging themobile phone, an external memory slot 950, and the like. Further, anantenna is incorporated in the housing 941. At least one of the housings940 and 941 includes the memory device or the signal processing circuitaccording to one embodiment of the present invention. Therefore, in thecase where supply of power is stopped for a short time, powerconsumption of the mobile phone can be reduced.

Further, the display panel 942 is provided with a touch panel. Aplurality of operation keys 945 which are displayed as images are shownby dashed lines in FIG. 25D. A boosting circuit by which a voltageoutput from the solar cell 949 is increased to be sufficiently high foreach circuit is also equipped.

In the display panel 942, the display direction can be appropriatelychanged depending on a usage pattern. Further, the display device isprovided with the camera lens 947 on the same plane as the display panel942, which enables videophone calls. The speaker 943 and the microphone944 can be used for videophone calls, recording and playing sound, andthe like as well as voice calls. Further, the housings 940 and 941 in astate where they are developed as illustrated in FIG. 25D can shift bysliding to a state where one is overlapped with the other; therefore,the size of the mobile phone can be reduced, which makes the mobilephone suitable for being carried.

The external connection terminal 948 can be connected to an AC adapterand various types of cables such as a USB cable, which enables chargingand data communication with a personal computer. Further, a large amountof data can be stored and carried by a storage medium inserted into theexternal memory slot 950.

In addition to the above functions, an infrared communication function,a television reception function, or the like may be equipped.

FIG. 25E illustrates a digital video camera which includes a main body956, a display portion A 955, an eyepiece 951, an operation switch 952,a display portion B 953, a battery 954, and the like. The main body 956includes the memory device or the signal processing circuit according toone embodiment of the present invention. Therefore, in the case wheresupply of power is stopped for a short time, power consumption of thedigital video camera can be reduced.

FIG. 25F illustrates an example of a television set. In a television set960, a display portion 962 is incorporated in a housing 961. Images canbe displayed on the display portion 962. Here, the housing 961 issupported by a stand 963. The housing 961 includes the memory device orthe signal processing circuit according to one embodiment of the presentinvention. Therefore, in the case where supply of power is stopped for ashort time, power consumption of the television set can be reduced.

The television set 960 can be operated by an operation switch of thehousing 961 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

The television set 960 is provided with a receiver, a modem, and thelike. With use of the receiver, general television broadcasting can bereceived. Moreover, the television set can be connected to acommunication network with or without wires via the modem, wherebyone-way (from sender to receiver) or two-way (between sender andreceiver or between receivers) data communication can be performed.

This embodiment can be implemented in appropriate combination with anyof the above embodiments.

This application is based on Japanese Patent Application serial no.2011-113359 filed with Japan Patent Office on May 20, 2011, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a firstinverter, a second inverter, a first transistor, a second transistor, athird transistor, a fourth transistor, a first capacitor and a secondcapacitor; wherein an input terminal of the first inverter iselectrically connected to an output terminal of the second inverter anda second node, and wherein an input terminal of the second inverter iselectrically connected to an output terminal of the first inverter and afirst node, wherein one of a source and a drain of the first transistoris electrically connected to the output terminal of the first inverter,wherein one of a source and a drain of the second transistor iselectrically connected to the output terminal of the second inverter,wherein one of a source and a drain of the third transistor iselectrically connected to the output terminal of the first inverter,wherein one of a source and a drain of the fourth transistor iselectrically connected to the output terminal of the second inverter,wherein the other of the source and the drain of the first transistorand a first electrode of the first capacitor are electrically connectedto a third node, wherein the other of the source and the drain of thesecond transistor and a first electrode of the second capacitor areelectrically connected to a fourth node, and wherein each of the firsttransistor and the second transistor comprises an oxide semiconductor.2. The semiconductor device according to claim 1, wherein the oxidesemiconductor comprises at least two kinds of elements selected fromindium, gallium, tin, and zinc.
 3. The semiconductor device according toclaim 1, wherein an off-state current density of the first transistorand the second transistor is less than or equal to 100 zA/μm.
 4. Asignal processing circuit comprising the semiconductor device accordingto claim
 1. 5. The semiconductor device according to claim 1, furthercomprising: a precharge circuit comprising a fifth transistor, a sixthtransistor and a seventh transistor, wherein a gate of the fifthtransistor is electrically connected to a gate of the sixth transistorand a gate of the seventh transistor, wherein one of a source and adrain of the sixth transistor is electrically connected to one of asource and a drain of the fifth transistor and the first node, andwherein one of a source and a drain of the seventh transistor iselectrically connected to the other of the source and the drain of thefifth transistor and the second node.
 6. The semiconductor deviceaccording to claim 5, wherein the precharge circuit is configured tooutput a first precharge potential to the first node and a secondprecharge potential to the second node.
 7. The semiconductor deviceaccording to claim 5, wherein a third potential is supplied to the gateof the fifth transistor, so that the fifth transistor, the sixthtransistor, and the seventh transistor are turned on.
 8. Thesemiconductor device according to claim 2, wherein a first potential issupplied to a gate of the first transistor and a gate of the secondtransistor, so that the first transistor and the second transistor areturned on.
 9. The semiconductor device according to claim 1, wherein asecond potential is supplied to a gate of the third transistor and agate of the fourth transistor, so that the third transistor and thefourth transistor are turned on.
 10. The semiconductor device accordingto claim 1, wherein one of a source and a drain of an eighth transistorincluded in the first inverter is electrically connected to one of asource and a drain of an ninth transistor included in the secondinverter.
 11. The semiconductor device according to claim 10, wherein asixth potential is supplied to the one of the source and the drain ofthe eighth transistor.
 12. The semiconductor device according to claim1, wherein one of a source and a drain of a tenth transistor included inthe first inverter is electrically connected to one of a source and adrain of an eleventh transistor included in the second inverter.
 13. Thesemiconductor device according to claim 12, wherein a seventh potentialis supplied to the one of the source and the drain of the tenthtransistor.
 14. The semiconductor device according to claim 1, wherein afirst data signal is input to the other of the source and the drain ofthe third transistor.
 15. The semiconductor device according to claim 1,wherein a second data signal is input to the other of the source and thedrain of the fourth transistor.
 16. The semiconductor device accordingto claim 1, wherein the third node stores a fourth potential held in thefirst node, and wherein the fourth node stores a fifth potential held inthe second node.
 17. A semiconductor device comprising: cells arrangedin matrix of m rows and n columns, the cells each comprising: a firstinverter, a second inverter, a first transistor, a second transistor, athird transistor, a fourth transistor, a first capacitor and a secondcapacitor; wherein an input terminal of the first inverter iselectrically connected to an output terminal of the second inverter anda second node, and wherein an input terminal of the second inverter iselectrically connected to an output terminal of the first inverter and afirst node, wherein one of a source and a drain of the first transistoris electrically connected to the output terminal of the first inverter,wherein one of a source and a drain of the second transistor iselectrically connected to the output terminal of the second inverter,wherein one of a source and a drain of the third transistor iselectrically connected to the output terminal of the first inverter,wherein one of a source and a drain of the fourth transistor iselectrically connected to the output terminal of the second inverter,wherein the other of the source and the drain of the first transistorand a first electrode of the first capacitor are electrically connectedto a third node, and wherein the other of the source and the drain ofthe second transistor and a first electrode of the second capacitor areelectrically connected to a fourth node, wherein each of the firsttransistor and the second transistor comprises an oxide semiconductor.18. The semiconductor device according to claim 17, wherein the oxidesemiconductor comprises at least two kinds of elements selected fromindium, gallium, tin, and zinc.
 19. The semiconductor device accordingto claim 17, wherein an off-state current density of the firsttransistor and the second transistor is less than or equal to 100 zA/μm.20. The semiconductor device according to claim 17 is used as aregister.
 21. The semiconductor device according to claim 17 is used asa cache memory.